InterCity 125



The InterCity 125 (originally Inter-City 125, or High Speed Train) is a diesel-powered passenger train built by British Rail Engineering Limited between 1975 and 1982. Each trainset is made up of two Class 43 power cars, one at each end and six to nine Mark 3 carriages. The name is derived from its top operational speed of 125 mi/h. Initially the sets were classified as Classes 253 and 254.

As of July 2018, InterCity 125s remained in service with CrossCountry, East Midlands Trains, Great Western Railway, London North Eastern Railway and Network Rail. Most operated by GWR and LNER will be replaced by Class 800, 801 and 802s by December 2019; Abellio ScotRail introduced these trains on its long-distance routes in October 2018.

Background
In the late 1950s and early 1960s, the British Transport Commission (BTC) was modernising its rail network. It wanted to increase intercity speeds so that railways could compete more effectively with motorways. The government was unwilling to fund new railways and the BTC focused on increasing line speeds by developing new trains and modifications to the infrastructure. A team of engineers was assembled at the Railway Technical Centre in Derby in the early 1960s, to design and develop an Advanced Passenger Train (APT) capable of at least 125 mph incorporating many features not previously seen on British railways—such as tilting to allow higher speeds on curves.

The APT project suffered repeated delays and in 1970, the British Railways Board (BRB) decided it was not sufficiently developed to modernise the railways in the short term and at the instigation of Terry Miller, Chief Engineer (Traction & Rolling Stock), the BRB authorised the development of a high-speed diesel train for short-term use until the APT was able to take over. An operational prototype was to be built by 1972.

Prototype
The prototype high-speed diesel train, which became the InterCity 125, was formed of a rake of passenger coaches between two power cars, one at each end. The decision to use two power cars was taken early in the project as design engineers calculated that the train would need 4,500 horsepower to sustain 125 miles per hour on the routes for which it was designed (the Great Western Main Line, Midland Main Line, and the Cross Country Route), and it was established that no "off-the-shelf" diesel engine was capable of producing such power. Another factor was that two locomotives operating in push–pull formation, would cause less wear on the rails than a single locomotive. The framework of the new locomotive, classified British Rail Class 41, was built at Crewe Works and transferred to Derby Litchurch Lane Works for completion. The design incorporated a driving desk around the driver, a sound-proofed door between the cab and the engine room, and, unusually, no side windows. The prototype was the first diesel locomotive in British railway history to use AC alternators in place of a DC generator, with the output converted to DC when used for traction.

The prototype train of seven coaches and two locomotives was completed in August 1972 and in the autumn was running trials on the main line. The following year, high-speed testing was undertaken on the "racing stretch" of the East Coast Main Line between York and Darlington. The set was reduced to two power cars and five trailers, and there was a concerted attempt to see how fast the train would go. On 6 June 1973 131 mph was reached, which was exceeeded as the days passed. By 12 June a world diesel speed record of 143.2 mph was achieved. The drivers believed that 150 mph was possible but the BRB issued instructions for the high speed tests to cease. It was believed at the time that this was because the BRB wanted to promote the APT as the future of high speed rail travel in the UK.

The fixed-formation concept was proven in trial running between 1973 and 1976, and British Rail built 27 production HSTs to transform InterCity services between London Paddington, Bristol, Cardiff and Swansea.

Production versions
The first production power car, numbered 43002, was delivered in late 1975 with a different appearance from the prototype. The streamlined front end lacked conventional buffers, and the drawgear was hidden under a cowling. The single cab front window was much larger than the prototype's, and side windows were included. There was no driving position at the inner end.

The train's appearance is the work of British designer Kenneth Grange who was approached to design the livery but under his own impetus decided to redesign the body, working with aerodynamics engineers. "It really was rather quite brutal, rather clumsy. I thought, 'Oh I'd like to get my hands on that', although the brief was nothing to do with the shape, absolutely not at all." He presented the new design to British Rail and persuaded them to adopt it.

After being withdrawn from GWR service, 43002 was acquired by the National Collection in September 2019 and was displayed at Locomotion.

An InterCity 125 consists of two Class 43 diesel-electric power cars, each powered originally by 2,250 bhp (1,678 kW) Paxman Valenta engines (although they have since been fitted with different engines), and a set of six to nine Mark 3 coaches.

Key features of the design are the high power-to-weight ratio of the locomotives (1678 kW per ~70-tonne loco), which were built for high-speed passenger travel, improved crashworthiness over previous models, and bi-directional running avoiding the need for the locomotive to run around at terminating stations. Until the HST's introduction, the maximum speed of British trains was limited to 100 mph. The HST allowed a 25% increase in service speeds along many of the lines on which they operated.

Lighter axle loading allowed the trains to travel faster than conventional services along lines not suited to full-speed running, such as the Edinburgh to Aberdeen line. Coupled with superior acceleration over older locomotives, this allowed substantial cuts in journey times. The increased speed and rapid acceleration and deceleration of the HST made it ideal for passenger use.

Introduction into service
Deliveries continued through 1976, and on 4 October a partial service of HSTs running at 125 mi/h began on the Western Region. The radical update of the standard BR livery on the power cars was complemented by the 'Inter-City 125' branding, which also appeared on timetables and promotional literature. By the start of the summer timetable in May 1977, the full complement of 27 Class 253 sets (253001–253027) was in service on the Western Region, replacing locomotive-hauled trains on the Bristol and South Wales routes. Passenger numbers rapidly increased due to the speed and frequency of the services. The displacement by HSTs of Class 50 locomotives to slower services effectively finished off the last Class 52 diesel-hydraulics by early 1977. The production of Class 254 continued through 1977 for East Coast Main Line services. British Rail had planned to fit uprated 2500 bhp Valenta engines to the longer HSTs, but the plan was shelved as the intensive running on the Western Region began to result in a high level of engine failures, often due to inadequate cooling; for a while, the WR power cars were derated to 2000 bhp. The Class 254s began to work important ECML expresses such as the Flying Scotsman from the summer timetable in May 1978. Within a year they had displaced the Deltics and reduced the London-Edinburgh journey time by up to an hour.

Production of HSTs continued until 1982 for services from London to the West Country on the Cross Country Route and on the Midland Main Line, serving destinations such as London, Bristol, Edinburgh, as far south as Penzance and as far north as Aberdeen and Inverness. The first Inter-City 125 train for Bristol left Paddington station, London on October 4, 1976 at 08:05. Ninety-five HST sets, including 197 Class 43 powercars, were built between 1976 and 1982. More Mark 3 trailer cars were built in the 1980s for the Western Region Class 253s, making them eight-car rakes in common East Coast and Midland Main Line services. During the 1990s only the Cross-Country sets remained as seven-car rakes, with just one first-class carriage.

Not only did the HST bring considerable improvements in service, British Rail began active marketing to support the train's introduction. The InterCity service was a great success for British Rail.

World records
The prototype InterCity 125 (power cars 43000 and 43001) set the world speed record for diesel traction at 143.2 mph on 12 June 1973. An HST also holds the world speed record for a diesel train carrying passengers. On 27 September 1985, a special press run for the launch of a new Tees-Tyne Pullman service from Newcastle to London King's Cross, formed of a shortened 2+5 set, briefly touched 144 mi/h north of York. The world record for the fastest diesel-powered train, a speed of 148 mph, was set by an HST on 1 November 1987, while descending Stoke Bank with a test run for a new type of bogie for use on Mark 4 coaches on the same route.

South West England and South Wales
On the Western Region, InterCity 125 trains (designated class 253) were introduced on services from London to Bristol and South Wales, and extended to most daytime services from London to Devon and Cornwall. Some South Wales services were extended to Milford Haven, Fishguard and Pembroke in West Wales. Maintenance was provided at Old Oak Common and St Philip's Marsh, and Laira also carried out maintenance after services to Devon and Cornwall were introduced in 1979.

British Rail Class 47 locomotives still operated cross-country services from Cornwall and South Wales to the North-East via the Cross Country Route, and London to the Midlands/Welsh Marches. Class 43s replaced them when the third batch of power cars was delivered in 2+7 formation with two first class coaches, a buffet car, and four second class coaches between two power cars. They were later expanded to a 2+8 formation, with an extra second class carriage. Great Western Trains was formed out of the privatisation of British Rail and operated the InterCity routes from London Paddington to the west of England. In 1998 FirstGroup acquired Great Western Trains and rebranded it First Great Western. InterCity 125s continued to work the same diagrams they had under British Rail, albeit in a different livery.



Great Western Railway used 43 HST sets to operate most intercity services from Paddington to Bristol, Bath Spa, Chippenham, Swindon, Cardiff, Swansea, Carmarthen, Cheltenham Spa, Oxford, Worcester, Hereford, Paignton, Plymouth and Penzance, and some commuter services to Westbury, Taunton and Exeter St Davids. In 2012 all First Great Western's intercity services were worked by InterCity 125 sets with the exception of sleeper services and some Cotswold Line services.

From 2005 the First Great Western HSTs were re-engined with MTU power units and the coaches were refurbished. Units for services in the M4 corridor/Thames Valley to Bristol, Hereford, Oxford, Exeter and Cardiff were converted to a high-density layout of mostly airline-style seats in standard class (only two tables per coach) to provide more seats for commuters. The remainder, for routes to Swansea and the West Country, included four tables per standard class coach.

The refurbished coaches had new seating (leather in first class), at-seat power points and a redesigned buffet bar. From 2010, one standard class carriage in each set had a Volo TV system, but this was removed in 2014.

Another change was made in 2014, when some first class coaches were converted to standard class or composite (half standard and half first class), leaving 1½ first class coaches per set. The first class coaches were refurbished in a more luxurious style, and many tables with one seat each side in first class were replaced by individual airline-style seats.

Eastern England / Scotland
On the East Coast Main Line, the InterCity 125 designated Class 254 was the staple stock after the retirement of the Class 55 Deltic locomotives in 1980–1982, until the introduction of InterCity 225 following electrification in 1990. They were concentrated on services from London King's Cross to Newcastle and Edinburgh Waverley, and to Glasgow Queen Street, Inverness and Aberdeen. In the months following the Penmanshiel Tunnel collapse in 1979, London to Scotland services ran via the Tyne Valley line from Newcastle to Carlisle then to Scotland via the West Coast Main Line. HSTs were also used on some services from London to Leeds, Bradford Forster Square, Cleethorpes, Hull and Scarborough.

The East Coast (ECML) formation was originally 2 + 8, increased to 2 + 9 in 2002 when extra stock became available. The ECML formation is two first-class coaches, one buffet (with 1st Class seating) and five (later six) standard-class coaches between the buffet and power cars. For a few years, formations included a TRUK (trailer restaurant kitchen) and buffet car, a TS (trailer second class) and TF (trailer first class) coaches, many formations were 4 × TS, TRUK, Buffet, 2 × TF. Nine trailer car units followed this formation, with the addition of a TS. 'Pullman' services replace a TS with an additional first-class coach.

After privatisation, InterCity 125s were operated by Great North Eastern Railway (GNER), alongside electric InterCity 225 units from London to Newcastle and Edinburgh, and beyond the electrified sections (or where British Rail Class 91s cannot operate due to route availability restrictions) services to Hull, Skipton, Harrogate, Inverness and Aberdeen.

In January 2007 the first of GNER's 13 refurbished HSTs was unveiled with coaches rebuilt to the same 'Mallard' standard as its InterCity 225 electric sets with similar seating, lighting, carpets and buffet cars. The power cars were upgraded with MTU engines. The first of the HST Mallards was in service by spring 2007. In 2007 the franchise was taken over by National Express East Coast (NXEC), which continued the re-engining programme begun by GNER, and completed the refurbishment of the fleet in March 2009. Two power cars were transferred to First Great Western early in 2009. The final Mallard-upgraded Mark 3 coaches entered service with NXEC in October 2009.

Following an announcement by National Express that it would not provide further financial support to NXEC, the franchise ceased on 13 November 2009, and the operation of the route returned to public ownership. As a result, the 13 sets were operated by East Coast from late 2009. East Coast introduced an InterCity 125 service to Lincoln in 2011. The InterCity 125 was replaced by the electric InterCity 225 on the line to Skipton when the electrical infrastructure was upgraded. Eight East Coast services per day in each direction used the InterCity 125. In 2012, 43072 (now 43272), 43074 (now 43274) were transferred to East Midlands Trains and received MTU engines. In April 2015, Virgin Trains East Coast took over operation of the InterCity East Coast franchise. All trains passed with the InterCity East Coast franchise to London North Eastern Railway in June 2018.

In 2006, Grand Central obtained six Class 43 power cars to operate its London-Sunderland passenger service via the East Coast Main Line. The service was due to begin in December 2006 although upgrade work to enable the coaching stock (which was formerly used for locomotive-hauled services and had a different electric heating/power supply system) to operate with Class 43 power cars was heavily delayed and therefore pushed the starting date back to 18 December 2007. HSTs 43084 and 43123 were the final operational Paxman Valenta power cars, being re-engined in 2010 with the same MTU engines as other units. While at the works being re-engined, Grand Central added the orange stripe that appears on their Class 180 units, re-painted the front ends (this making them look more like the non-buffered HSTs), and re-numbered the power cars into the four-hundreds. Grand Central's HSTs were cascaded to East Midlands Trains at the end of 2017.

In February 2019 Hull Trains commenced using a First Great Western set between London King's Cross and Hull due to ongoing reliability issues with its Class 180s. A month later they introduced an additional set following further reliability issues.

Midland Region
On the London Midland Region, InterCity 125 trains were introduced later than on the other regions. They initially appeared on the former Midland Railway route from London St Pancras to Sheffield and Nottingham. Although they were initially not permitted to exceed 100 mi/h on any part of the route, they still delivered time savings compared with the loco-hauled trains they replaced.

The Midland Main Line received a series of speed improvements over the next two decades, until it became possible for HSTs to run at up to 110 mi/h on some sections. An upgrade to the full 125 mi/h was proposed by British Rail in the early 1990s, but because of privatisation this did not happen. However line improvements were completed in time for the spring 2014 timetable change, which has permitted 125 mph running on some sections of the line and higher top speeds on others.

Most long-distance services on this route have been transferred to new Class 222 Meridian diesel-electric multiple units, although many London services from Nottingham still use the InterCity 125, as do all services from London St Pancras to Leeds. Midland Mainline inherited HSTs from BR after privatisation and operated them on its primary services at up to 110 mph.



43089 also was returned to work on the mainline after being used in an experimental programme conducted by Network Rail and Hitachi. 43072, 43074 was transferred to East Coast in 2012. Currently, 30 are in service with East Midlands Railway. Since December 2013, InterCity 125 sets have been permitted to operate at speeds of up to 125 mph on certain parts of the routes from London St Pancras to Leeds and Nottingham.

Cross-Country Route
Post privatisation the Cross-Country Route was operated by Virgin CrossCountry, who replaced the InterCity 125 trains in the period 2002–2004 with Voyager high-speed DMUs.

The majority of the former Virgin CrossCountry fleet went into storage for several years but a small number moved to Midland Mainline to supplement its fleet.

In 2007 the franchise passed to Arriva CrossCountry. Because of overcrowding, Cross Country reintroduced five HSTs to supplement its Voyagers. In late September 2008 CrossCountry refurbished its first HST set. The coaches were refurbished to a similar "Mallard" standard as GNER trains, though their interior is in burgundy and there are fewer tables. They also differ from the East Coast sets by having electronic seat reservations, and the buffet car has been removed, with all catering provided at-seat from a catering base in coach B. Most of the carriages are rebuilt from loco-hauled Mark 3s. The refurbishment was carried out by Wabtec, Doncaster Works.

Each set has had a TS removed (now 2 power cars + 7 coaches).

CrossCountry operates HSTs to the following destinations:


 * Plymouth
 * Leeds
 * Edinburgh
 * Dundee
 * Glasgow
 * Newquay (summer weekends only)
 * Penzance (summer weekends only)
 * Paignton (summer weekends only)

West Coast and North Wales
Virgin Trains West Coast HSTs regularly worked out of London Euston and Birmingham International to Holyhead and Blackpool North. They also worked some Euston to Manchester Piccadilly services. Virgin's HST's were re-deployed in May 2004. Due to there being numerous curves on the West Coast Main Line, the trains were not permitted to exceed 110 mph on any part of the route.

When the West Coast Main Line was upgraded by Network Rail in the 2000s, it became necessary to operate diversionary routes whilst work was going on. As a result, Midland Mainline was asked by the then Strategic Rail Authority (SRA) to operate services between London and Manchester via the Midland Main Line and Hope Valley Line into London St Pancras while West Coast Main Line works took place. In a temporary operation dubbed Project Rio, a large percentage of the stored Virgin CrossCountry power cars were overhauled and returned to service in an enlarged Midland Mainline fleet. Ending on 10 September 2004, the Project Rio fleet was gradually disbanded, with power cars moving to First Great Western and GNER.

ScotRail
Twenty-six HST sets, each with four or five carriages, are to move from Great Western Railway to Abellio ScotRail after being refurbished by Wabtec at Doncaster with new interiors, controlled emission tanks and automatic sliding doors. They will operate on services from Edinburgh and Glasgow to Aberdeen and Inverness. The first set entered service in October 2018, with units entering service  into Glasgow for the timetable change in December being unrefurbished, still in debranded GWR( both blue and green) liveries. ScotRail's HST fleet operates under the brand name Inter7City.

Network Rail
One HST set is in service with Network Rail, painted in then-Departmental yellow, and often referred to as the "flying banana" (a nickname that was originally applied to the whole class because when first introduced by BR they wore a predominantly yellow livery). The set is the New Measurement Train.

Another single engine, 43089, was used in tests on hybrid battery powered vehicles in collaboration with Hitachi. It has since been returned to normal service with East Midlands Trains.

Numbering and formation
Because they were fixed formation trains, British Rail considered the Inter-City 125 sets to be diesel multiple units. They were allocated British Rail Class 253 (2+7 sets allocated to Western Region depots for use on Western Region and Cross-Country services) and Class 254 (2+8 sets allocated to the Eastern and Scottish Regions for use on the East Coast Main Line), the prototype train having been Class 252. Therefore, each set was allocated a set number (253 xxx or 254 xxx), which was carried on the front of the power cars. Individual vehicles were numbered in the 4xxxx series (see table below), and, because they were regarded as multiple unit vehicles, also had regional prefixes according to their allocated depot (E for Eastern Region, SC for Scottish and W for Western); this included the power cars as well as the trailers.

With power cars often requiring maintenance more frequently than the trailer cars, power car swaps soon began to take place; there were a few spare power cars to allow for this. This often resulted in different set numbers being displayed at each end of the same train. As a result, during the early 1980s the power cars began to be regarded as "loose", and the use of set numbers for the whole train was abandoned. The trailer cars remained in fixed formations, however, and still allocated a set number of sorts, although that was not displayed anywhere.

As sectorisation began to take hold during the mid-1980s, the use of regional prefixes on coaches and multiple unit vehicles was discontinued. At about the same time it was decided to reclassify the InterCity 125 trains (the hyphen having been dropped by the new InterCity sector) as locos and coaches. To avoid renumbering the power cars, they became British Rail Class 43 diesel locos, although a space was never inserted between the second and third digits (as was common practice on other locos at the time, e.g. 47 401).

The vehicle types used to form High Speed Trains are listed below:

The 197 production series power cars were numbered 43002-43198. 43001 was applied to the second of the two prototype power cars, while the first of the pair (now preserved as part of the National Collection) became 43000 – unusual because BR TOPS classification numbered its locomotives from 001 upwards (this was because it was not, at the time, classified as a locomotive). Subsequently, on fitting of new engines, power cars operated by the InterCity East Coast and Cross Country franchisees have been renumbered in the 432xx or 433xx series (by adding 200 to their serial numbers), while Grand Central also changed the third digit of its power cars to 4 (by adding 300 or 400).

The renumbering of the 400xx series catering vehicles in 1983 was to avoid their numbers clashing with the Class 40 diesel loco fleet (numbered 40 001 to 40 199) when BR's loco (TOPS) and coaching stock number series were merged.

In 2002, Class 255 was allocated for the reformation of some HST power cars and trailers into semi-fixed formation trains, to be known as Virgin Challenger units, for use by Virgin CrossCountry. These formations would have had power cars sandwiching one Trailer First, a Trailer Buffet, two Trailer Seconds and a Trailer Guard Second. These plans came to naught as the Strategic Rail Authority planned to transfer most of the stock to Midland Mainline for its 'Rio' services between London and Manchester.

Livery




The original "Inter-City 125" livery was blue and grey, with a yellow front to improve visibility which continued down the side of the power cars. This was the livery recorded in British Rail's corporate identity manual of August 1977.

The second livery had mostly grey power-cars with a white band along the middle, yellow underneath the white band, with the InterCity colours (cream, red, white, brown) for the parcel compartment of the power cars and the coaches.

There was brownish-grey, dark grey (almost black) around the windows with a red and white stripe below the windows, and retaining the yellow bands on the power cars. The final variant of this livery saw the yellow side-bands replaced with white and did not feature the British Rail name or logo: it carried the new sector branding Intercity logo in serif type and an image of a flying swallow. This is commonly referred to as "InterCity Swallow" livery, and was applied to other locomotives in the sector.

After the privatisation of British Rail, train operating companies painted the HSTs in their own colour schemes, with some lasting longer than others.

Two of Great Western Railway's powercars have been repainted into heritage livieries; 43002 has repainted into original InterCity 125 Blue & Yellow livery whereas 43185 has been repainted into InterCity Swallow livery.

Public reaction
The Intercity service proved an instant hit with the British public. By the early 1980s the HST had caught the travelling public's imagination, thanks in part to a television advertising campaign fronted by the since-disgraced entertainment personality Jimmy Savile, together with the advertising strap-line "This is the age of the train". British Rail enjoyed a boom in patronage on the routes operated by the HSTs and InterCity's revenues increased, although the total was not enough to remove the need for subsidies to British Rail.

International attention
The success of the HST has had significant international impact. Foreign press for decades observed and praised the speed and quality of the service.

The InterCity 125 was used as a case study for evaluating the potential for a high-speed rail system in California. In Australia, the HST was used as the base for developing the XPT, in cooperation with British Rail.

Scale models
There have been many model and toy guises of the IC125. One of the first in the UK was by Hornby Railways, which launched its first model version in 1977. This model was supplied with an incorrect length Mk3 coach which was shortened to allow the model to reliably negotiate the smallest radius curves. This was done by removing one of the 8 side windows rather than scaling the whole length. It was later released in InterCity 'Swallow' livery, Great Western green-and-white, Midland Mainline and Virgin Trains. Lima released its version of the IC125 in 1982, of which the Mark 3 coaches were correct to the lengths of the real-life coaches and included the guard's coach. Hornby eventually followed suit in the late-1990s, when its short Mark 3 coaches were replaced by correct scale length ones but omitted the guard's coach. Hornby released a totally new version of the InterCity 125 power cars in late 2008. Graham Farish were the first to produce a HST in N gauge, recently Dapol have produced another N gauge model of the train. Railway Shop (Hong Kong) produces a T gauge model (1:450 scale).

Damaged vehicles and accidents
Three Class 43 locomotives have been written off in railway accidents, all of which occurred on the Great Western main line. 43011 was written off in the 1999 Ladbroke Grove rail crash, 43019 was written off after colliding with a car at Ufton Nervet in 2004, and 43173 was scrapped at Pig's Bay in Essex after heavy damage in the Southall rail crash of 1997. In each of these cases, the damage was to the leading power car – the trailing power cars suffered limited or no damage, and were returned to service. At Ladbroke Grove and Ufton Nervet the accidents were ultimately caused by factors not involving the HST sets or their drivers, although the set involved in the Ladbroke Grove crash had a faulty AWS system; however, the Southall accident was due to the HST colliding with a goods train which was entering Southall Goods Yard, crossing the main lines. The immediate cause of the crash was the result of the driver of the HST passing a red signal without stopping. In addition, the leading power car of the set had a faulty Automatic Warning System which if operational would have alerted the driver to his error and possibly prevented the accident. Following investigation, this system has since been required to be kept operational and switched on for all use of the InterCity 125 fleet.

Re-engining and refurbishment


In 2005, the train leasing company, Angel Trains, initiated and led an industry-wide programme to replace the 30-year-old Paxman Valenta engines in the HST power cars with new MTU 16V 4000 engines. The upgrade, which was part of a £110 million total investment made by Angel Trains on its fleet of High Speed Trains, included the re-powering and refurbishment of 54 HST power cars: those then on lease to GNER (now London North Eastern Railway) (23), First Great Western (26) and CrossCountry (5). Virgin CrossCountry planned a similar project in the early 2000s, but with the collapse of the programme the upgraded trainsets were sold along with their unupgraded stablemates.

Additionally, many operators undertook some refurbishment of the Mark 3 carriages in the early 2000s. In view of the delay and change of direction of the HST2 programme, operators began to refurbish their HST fleets in 2006 – both by remotoring with the more modern MTU4000 diesel engine, and by refurbishing the carriage interiors. It was anticipated that these overhauls would give the HST at least another 10 years in front-line service.

Replacements
The first partial replacement of HSTs occurred from 1988 on the East Coast Main Line, with the introduction of the InterCity 225 when the line to Edinburgh was electrified. Some were retained for services to Aberdeen, Inverness, Skipton, Bradford and Hull.

As the Intercity 125 fleet has become old compared to most stock used in passenger service, it has been recognised that it is near the end of its service life. More recently, HSTs have been replaced (or augmented) by high-speed DMUs such as the Voyagers and the UK express version of Alstom's Coradia. These DMUs have better acceleration than the HST due to a higher power/weight ratio, with greater efficiency and braking performance in addition.

In 2005 the initial concept of HST2 was rejected by the government and the rail industry as a like-for-like replacement for the HST fleet. In the light of this rejection, in 2006 existing operators turned to refurbishments of the InterCity 125 trains. Nevertheless, the HST2 concept was expanded and replaced by the Intercity Express Programme, with proposals for a joint replacement of both HST and InterCity 225 trains. The eventual successor to the two Intercity fleets is the Hitachi Super Express, comprising three classified types of fixed-rake formations: two electro-diesel types, the Class 800 sets and the Class 802 sets and the electric multiple unit Class 801 sets.

On the Greater Western franchise, the first HST was returned to its leasing company in September 2017, and by June 2019, all sets on Intercity routes were withdrawn in favour of the Class 800 and the Class 802 sets.

Sewage discharge
Legally in the UK, train operators are allowed to discharge up to 25 litres of untreated waste at a time on to the track. Most Mk3 carriages have no toilet tanks, discharging directly onto the track. In the 2000s both the RMT trade union and politicians were concerned at the environmental impact of this legacy issue. The problem was first raised in 2003 after Railtrack staff at Nottingham abandoned local clean-up and then track maintenance procedures due to an excessive buildup of sewage waste in the area. In 2006 the RMT agreed waste-tank and clean-out developments at Northern Rail's Heaton depot in 2006 with GNER, plus new clean-out procedures at all other depots, to solve an ongoing dispute over the previous 18 months.

By 2011, the European Union had started a formal investigation to see whether trains composed of such carriages were breaking EU environmental and health laws, although the Environment Agency confirmed that train companies claimed special exemptions to dump waste along the tracks. In 2013, transport minister Susan Kramer branded the practice "utterly disgusting" and called on the industry to take action. ATOC responded by stating that, as all new vehicles had to be fitted with compliant toilet tanks, with withdrawal of the HSTs by the end of 2017 the problem would be solved. HSTs remained in operation after 2017, but sliding-door conversions of CrossCountry's fleet and the shortened sets for GWR and ScotRail have all included the fitting of controlled emission toilets, while ScotRail is seeking to fit retention tanks to its remaining unrefurbished carriages.

Future
Great Western Railway are to retain 24 powercars and 48 carriages to form 11 four-carriage sets for use on local services between Cardiff and Penzance. The carriages will be fitted with automatic doors and controlled emission tanks at Wabtec, Doncaster.

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In 1982 the State Rail Authority in Australia placed the first of what would ultimately be 19 power cars and 60 carriages into service.

Built locally by Comeng and ABB Transportation, the HST design was significantly modified, with the power cars being 50 cm shorter, the Paxman Valenta engine downrated from 2,250 to 2,000 bhp, gearing lowered for a top operating speed of 160 km/h, suspension modified to operate on inferior track, and air filters and the cooling system modified to cater for hotter and dustier Australian conditions. However, the XPT is theoretically capable of reaching speeds of 200 km/h. A different light cluster was fitted along with three high-beam spotlights mounted to the roof. The carriages were based on a Budd design, with the British Rail Mark 3s considered unsuitable.

In 1986 agreement was reached to build a fleet of XPTs for the State Railway of Thailand. To allow it to be built to the narrower gauge and retain the same fuel capacity, it was proposed to extend the power cars by 2.7 metres to 20 metres and mount them on Bo-Bo-Bo bogies. The negotiations were sufficiently advanced for the Prime Minister of Thailand to announce it on television, however the Australian Department of Trade withdrew its support at the last moment and the deal fell through.