British Rail Class 43 HST (Intercity 125)


The British Rail Class 43 High Speed Train (HST) or Intercity 125 is a high speed diesel locomotive, capable of speeds of up to 148 mph. Powered by a Paxman Valenta prime mover, the HST holds the record of the fastest revenue diesel passenger train, when it hit 144 mph between Newcastle, and London King’s Cross, on the East Coast Main Line (ECML).

Due to the lack of government funding to upgrade the existing rail line, including electrification, the British Transport Commission developed a plan of action to facilitate high speed trains while using the existing rail line. They then decided that the best course of action was to implement a diesel train, and perform low cost upgrades to the existing rail line. Throughout the course of ten years, a prototype was built;  the original Class 41 HST. This locomotive formed the foundation of how the final product would operate. Introduced in 1975, the HST was slated to replace the current diesel power on British Rail’s main line passenger services. Due to its successful design, the HST became popular for many high speed routes in British Rail’s Western, Scottish, Eastern, and London Midland operating areas.

Hugh Llewelyn photo


The Class 43 HST was powered by a Paxman Valenta prime mover, operating at 1500 rpm, and producing 2,250 horsepower. The low axle load, made possible by the high speed diesel engine, allowed for quick acceleration, and reduced wear on railway lines and the train’s running gear.

The HST was delivered in the original “Intercity 125” paint scheme,which consisted of a blue and gray color combination. Later schemes included the addition of grey with a white stripe alongside, with the bottom half being painted yellow.

The lightweight design and performance can be attributed to its designer, Kenneth Grange, who engineered the high speed aerodynamic capabilities of the train. The Class 43, called the “Intercity 125” under British Rail, consisted of two HST Class 43 diesels on either end, with up to nine Mark 3 coaches.

At the time the trainset was introduced, trains in Britain were limited to 100 mph on most routes, as well as many operating at much slower speeds. Due to the frequency of service and quick travel times, many routes served by the HST’s gained ridership. The HST took over many of the services previously served by the Class 50, built by English Electric, and the Class 52 Western Hydraulics. The Intercity 125 was considered a success for British Rail, and greatly improved the efficiency and reliability of service.

Duncan Cotterill/railography


During the engineering phase of the HST, a prototype called the British Rail Class 41 was constructed. Engineered at the  Railway Technical Centre, and later constructed at British Rail Engineering Limited (BREL), was powered with a Paxman Valenta prime mover, and featured a unique front fascia that included an aerodynamic design.

Two prototype power cars were developed 41001 and 41002, and were used to test its high speed ability on multiple rail lines including the East Coast Main Line (ECML), and the Great Western Railway, on tests towards Cardiff. Although short-lived, the prototype HST was the foundation of a revolutionary train that would still be seerving Britain almost five decades later.

Today, the prototype is restored and is in operating condition thanks to the 125 Group of volunteers. In 2014, it hauled a special excursion called “The Screaming Valenta”. The restored locomotive received a Paxman Valenta RP200L, the original prime mover in which it attained upon delivery. Upon restoration, it was reclassified as a Class 43/9, in order to operate on Network Rail’s trackage.

Currently, the prototype HST is preserved in operating condition at the Great Central Railway in Ruddington, in cooperation with the 125 Group.

HST Prototype Class 41
Hugh Llewelyn photo


In the mid-2000s, refurbishment of the HSTs were considered, as the class was almost over thirty years old by this time. The Paxman Valenta prime movers were effective, however, out of date for current standards, especially for emissions. Rail companies who operated the HSTs considered a new prime mover. Considering the versatility and speed of the HST, rail operators were not yet ready to look towards other options, so, repowing of the HST seemed the most effective solution.

The chosen prime mover replacement for the class was constructed by MTU, a German company specialising in commercial engines. MTU was founded by William Maybach, the same individual, who’s company pioneered the prime movers for the Class 52 Western hydraulics. These new prime movers offered improved emissions, improved fuel efficiency and were much quieter than the Valentas. The chosen model of prime mover was the MTU V16 4000, and by December 2010, all HST on the national network were fitted with MTU prime movers.

To send off the Valenta engine that had served Britain for many years, a “Farewell to the Valenta” excursion was held in December of 2010, and operated by Grand Central, that traversed the East Coast Main Line one last time.

Current Operations

Currently, many rail operators in the United Kingdom use the HST in daily service:

The largest number of HSTs belong to the Great Western Railway (GWR), as they currently own sixty-four examples. These are used on services from London Paddington westward to Cardiff Central. However their days are numbered as they are being replaced by the new Hitachi Class 800 train sets.

Cross Country operates ten examples of the HST, mainly serving the west and east Midlands lines, as well as southwest, northeast, and Yorkshire and the Humbler.

East Midlands trains operates thirty examples of the HST. These examples are unique as during re-powering they received Paxman VP185 prime movers, instead of the standard MTU prime mover. These operates on the Midlands Mainline, including East Midlands and Yorkshire and the Humbler.

Scotrail operates fifty-four examples of the HST. Recently acquired from Great Western Railway, whose HSTs were displaced due to the arrival of the Class 800 locomotive. These serve on the Scotrail routes between Edinburgh and Glasgow, with service to Inverness and Aberdeen.

London & North Eastern owns thirty-two HSTs for service on the East Coast Mainline. These sets are commonly used for the non-electrified portions of its operating area, however, are commonly seen supplementing the Class 91 “Intercity 225”, on many occasions.

Network Rail, the owner of most railways in the country, uses three specially fitted HSTs for inspection of the West Coast Mainline, ensuring that the line is maintained properly in order to run high speed tilting trains, such as the Class 390 “Pendolino, and the Class 221 “Super Voyager”, both operated by Virgin Trains. These HSTs are equipped with lasers and other equipment to scan track infrastructure. A report is then sent to the computers located within the train.

Duncan Cotterill/railography


Due to the HST being almost five decades old, replacements for the HST are being built. The Hitachi built Class 800 diesel multiple unit (DMU). The Class 800 has already been introduced on the Great Western Railway, and has displaced many of their HSTs. This effort is led by the Intercity Express Programme, who’s goal is to eventually phase out the Class 43 HST and the Class 91 electric locomotives, currently serving the East Coast Main Line under London & North Eastern Railway (LNER).

The Class 800 is a dual mode train set, able to run on either diesel or electric power gathered from overhead wires. The Class 800 offers increased fuel efficiency, and the versatility needed for the network. Even with the introduction of the Class 800, HSTs will still see service for almost another decade, serving passengers on the line between Devon and Cornwall due to the Class 800s incapability to traverse the sharp radius of the rail lines’ curves.


The record for the fastest diesel passenger train is held by the HST to this day. Reaching 143.2 mph without passengers on June 12, 1973. Additionally, it became the fastest diesel passenger train in revenue service on September 27, 1985, when it was on a publicity run, where it hit 144 mph. The HST rejuvenated high speed rail in Britain, as it improved the efficiency and reliability of the rail network.


Wheel ConfigurationBo’Bo’
Wheel Diameter3 ft 4 in
Length17.79 metres
Width2.74 metres
Weight70.25 tonnes
Fuel Capacity990 imperial gallons
Prime MoverPaxman Valenta 12VP185

MTU 16V4000 R41R

AlternatorValenta VP 185

Brush BA1001B

Traction MotorsGeneral Electric Company G417AZ

Brush Traction TMH68-46

Max SpeedRevenue Service: 125mph

Highest Recorded Speed: 148mph

Horsepower2,250 hp
Tractive EffortMax: 17,980lbf

Continuous: 10,340lbf

Route Availability5


Lifelong Rail Enthusiast and Owner of Worldwide Rails

Recent Posts