[citation needed] 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.

[1] 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 aging multiple units, particularly due to the necessity of handling and removing hazardous materials such as asbestos.

[2] In the concept stage, two separate approaches were devised, one involving a so-called railbus that prioritised the minimisation of both initial (procurement) and ongoing (maintenance & operational) costs, while the second was a more substantial DMU that could deliver superior performance than the existing fleet, particularly when it came to long-distance services.

[2] The initial specification developed for the latter type was relatively ambitious for the era, calling for a maximum speed of 90 mph (145 km/h), a rate of acceleration compatible to contemporary EMUs, the ability to couple/work in multiple with existing EMUs, facilitate through-access for passengers, feature pressure ventilation, the ability to assist another failed unit, and to comprise either a three or four-car consist.

However, to deliver the performance specified, it was found that relatively expensive equipment had to be used, particularly to provide sufficient speed, acceleration, and through-passenger access; it also had maintainability problems due to space limitations.

Despite these shortcomings, it was recognised that a production fleet that was assembled from proven components would possess both a greater reliability level and lower maintenance costs; it was forecast to achieve an availability rate of 85 percent.

[2] For an operational perspective, it was intended that the DMU could be assembled akin to building blocks, comprising between two and four cars that may or may not be outfitted with various passenger amenities such as toilets and luggage spaces.

A relatively constrained timetable of only 18 months between the date of order to delivery of these prototypes was also specified; this has been attributed as having restricted manufacturers to overwhelming lean towards existing industrial practices for their submissions.

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.

The railway engineering company Metro-Cammell also bid, offered its own design that employed rivetted aluminium construction; this feature was attributed as enabling a meaningful reduction in weight over conventional methods.

For the era, the Class 150 provided exceptional ride quality; it also was fully compliant with BR's 50 percent engine-out performance requirements.

Changes from the Class 150 included discarding openable windows in favour of fully-sealed units, while the external doors were relocated into vestibules at the end of each coach to reduce internal noise levels.

[2] Furthermore, the coaches were stretched to provide more internal volume, substituting the two-by-three seating arrangement of the Class 150 with a more roomy two-by-two counterpart.

Currently operated by: The Class 158 "Express Sprinter" units were built from 1989 to 1992 by BREL at Derby Litchurch Lane Works to replace elderly 'heritage' DMUs and locomotive-hauled passenger trains.

In the 1990s, Strathclyde Passenger Transport was looking to update the fleet that was used to operate commuter and suburban services in and around Glasgow, with a new Sprinter type intended to be procured, which became Class 157.

However, due to British Rail's financial limits at the time leading up to privatisation, these units were not ordered, with Class 170s eventually being procured.