By the beginning of the 1980s, British Rail (BR) operated a large fleet of first generation DMUs, which had been constructed in prior decades to various designs.

[10] While formulating its long-term strategy for this sector of its operations, British Rail planners recognised that there would be considerable costs incurred by undertaking refurbishment programmes necessary for the continued use of these ageing multiple units.

However, it was found to be expensive, and it was recognised that a production model assembled from proven components would possess greater reliability and lower maintenance costs; an availability rate of 85 percent was forecast.

[11] Specifically, it was decided to drop the top speed from 90 to 75 mph (145 to 120 km/h), as testing had revealed the higher rate to deliver no perceivable improvement in journey times due to the typically short spacing of the stations the type was intended to serve.

[11] The bid submitted by British Rail Engineering Limited (BREL) was heavily based on its successful Class 455 EMU, sharing its body and the majority of its running gear, albeit equipped with two different power trains.

Studies showed coaches could be stretched, providing more internal volume and thus enabling the somewhat cramped two-by-three seating arrangement of the Class 150 to be substituted with a more roomy two-by-two counterpart.

[11] It was identified that this would result in a weight increase and thus a decreased power-to-weight ratio, but it was determined that the performance of the proposed DMU was only slightly lower, and it could achieve similar journey times across the intended cross-country routes as the Class 150.

Specific changes include the bodyshell being primarily composed of steel instead of aluminium; the deliberate decision was made to model the cab design on the earlier Class 150 was allegedly taken to ease union acceptance.

[citation needed] Each coach is powered, being outfitted with a single six-cylinder Cummins NT855-R5 diesel engine coupled to a Voith T211r hydraulic transmission and Gmeinder final drive units.

Units operated by Abellio ScotRail have additionally been fitted with door-control panels near the centre sets of doors for the convenience of the guard.

[12] In the early 1990s, British Rail was looking to save costs on rural routes, and decided that operating two-car trains was too expensive.

[22] In December 2014, unit 156478 was written off by Angel Trains and sold to Brodie Leasing after being damaged by floodwaters on the Glasgow South Western Line.

This refurbishment included a larger toilet, a dedicated wheelchair area and brand new interior upholstery with new seats similar to those found on the Class 385.

[citation needed] Having originally been based in the East Anglia region but later transferred away, in early 2005 they returned when One received nine from Central Trains in exchange for a similar number of Class 150s.

Despite being overhauled by Railcare Wolverton in 2012/13, including work to make them comply with disability legislation, all were replaced by Class 755s with the last day of service being 29 January 2020.

[15] Following privatisation, both Arriva Trains Northern and First North Western operated Class 156s and implemented their own refurbishment programmes.

[citation needed] In 2011, fourteen Class 156s were fitted with GPS as a trial for Northern Rail, being tested on the Esk Valley line.

[32] In January 2015, Northern Rail began to hire 156s to First TransPennine Express to operate Manchester Airport to Blackpool North services.

[40] The transfer did not proceed after the Scottish Government refused to sanction the move, and the rest of the fleet were reliveried into Central Trains' colours between 2003 and 2005.

[14][15] In December 2022, four units were sent for storage at Barrow Hill, with East Midlands Railway stating that they will be moved to Ely Papworth sidings at a later date.