[8] 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.

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.

[9] 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.

[9] 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.

[9] Each vehicle is fitted with an underfloor-mounted Cummins NT855-R5 turbo-diesel engine, producing up to 213 kW (285 hp), driving both axles of the inside-end bogie via a Voith hydrokinetic transmission.

The units were the first BR DMUs to be furnished with sliding-plug automatic doors which closed to provide a smooth bodyside rather than sliding back into the bodyshell (the system used with 150s and 151s).

The conversion to single car units was notable for not requiring any external doors to be moved, although this has led to the No.2 cab being unusually cramped, despite being extended into the vestibule area.

[citation needed] The previously mentioned seven remaining Class 155 units, which are numbered 155341 to 155347, were constructed in 1988 for West Yorkshire Passenger Transport Executive (WYPTE) for their MetroTrain services, and have continued to serve in their original formation across multiple decades.