The InterCity 125 (originally Inter-City 125) or High Speed Train (HST) is a diesel-powered high-speed passenger train built by British Rail Engineering Limited between 1975 and 1982. A total of 95 sets were produced, each comprising two Class 43 power cars, one at each end, and a rake of seven or eight Mark 3 coaches. The name is derived from its top operational speed of 125 mph (201 km/h). At times, the sets have been classified as British Rail Classes 253, 254 and 255. British Rail (BR) initially developed the HST as an interim measure in the early 1970s, as delays and cost concerns began to threaten their primary high-speed train project, the Advanced Passenger Train (APT). The HSTs are now widely considered to be among the most successful trains to have operated on the British railway network, both in terms of their initial impact and their longevity: their introduction into service between 1976 and 1982 resulted in significantly reduced journey times, and large increases in patronage on the routes on which they were operated. The trains proved to be a reliable workhorse, remaining in front-line service for decades. The first withdrawals began in 2017, 41 years after they were introduced. As of December 2025, InterCity 125s remain in service with ScotRail, with one used as an engineering train with Network Rail. The HST design became the basis for an Australian variant, the Express Passenger Train (XPT), which entered service in New South Wales in 1982.
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 governments of the time were unwilling to fund dedicated high speed rail infrastructure, as happened in other countries, and so the BTC instead focused on developing new trains which could operate at higher speed using existing 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 miles per hour (201 km/h) 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 would not be sufficiently developed to enter public service until well into the next decade, so a stopgap solution would be needed to reduce journey times in order to compete effectively with other modes of transport. At the instigation of Terry Miller, Chief Engineer (Traction & Rolling Stock), the BRB authorised the development of a high-speed diesel train using tried and tested conventional technology for short-term use, until the APT was able to take over. An operational prototype was to be built by 1972.
Development
Concept and design The high-speed diesel train, which became the HST or InterCity 125, was formed of a rake of Mark 3 passenger coaches between two streamlined power cars, one at each end. Each power car was fitted with a Paxman Valenta diesel engine which could produce 2,250 horsepower (1,680 kW). This engine was chosen due to its light weight, and consequent high power-to-weight ratio. 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 (3,400 kW) to sustain 125 mph on the routes for which it was designed (the Great Western Main Line, East Coast 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.
British Rail had used a similar idea of a train with integral power cars at each end a decade earlier, with their Blue Pullman sets; although these trains were not ultimately commercially successful, they established the validity of the idea, and are often seen as forerunners of the HSTs. The concept had several advantages; firstly, a power car at each end allowed the train to be driven from either end in push–pull formation with the power cars linked by electronic control systems, and therefore reverse direction without the need for a locomotive to be run around at terminus stations, secondly, the train could run with only one power car operational, though at reduced speed, therefore allowing a journey to continue in the event of a breakdown, and thirdly, it avoided the underfloor diesel engines found in diesel multiple units, therefore avoiding noise and vibration for passengers. Another factor was that two locomotives operating in push–pull formation, would cause less wear on the rails than a single locomotive. One of the design requirements for the HST was that it should exert no more force on the track at 125 mph, than a Class 55 'Deltic' locomotive at 100 mph, and each power car of the HST weighed a modest 70 tons which allowed it to meet this requirement. One of the key design requirements for the HSTs was that they should be able to run at 125 mph on existing infrastructure. In order to achieve this, they had to be able to come to a stop from 125 mph within existing signal spacings, which required a high performance braking system to be developed. This was achieved by a high performance disc brake system, in place of the clasp brakes used on traditional stock.
Prototype
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