[6] 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.

[7] 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.

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 per cent.

[7] 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.

A rival bid was also submitted by British Rail Engineering Limited (BREL) that was heavily based on its successful Class 455 EMU, sharing its body and the majority of its running gear.

[7] The Class 151 comprised an aluminium-body that was principally rivetted together; this feature was attributed as enabling a meaningful reduction in weight of 3.7 tonnes over conventional construction methods.

[7] The type was designed to achieve a maximum speed of 75 mph (120 km/h), and was capable of attaining a relatively high rate of acceleration, being aided in this latter aspect by their lightweight construction.

[1] Each engine drove a hot-shift Twin Disc transmission, which in turn powered the axles of one bogie on each car via a Cardan shaft and Gmeinder final drive units.

[7] As a consequence of their non-standard nature, it was difficult to acquire support and spares for the bogies and several other unique features present on the units, a factor which allegedly heavily contributed to their withdrawal in 1989.

[7] During the trial, members of the public praised the Class 151 for its favourable ride quality; although some vibration was experienced on older sections of jointed track.