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. While formulating its long-term strategy for this sector of its operations, BR 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. In light of the high costs involved in retention, planners examined the prospects for the development and introduction of a new generation of DMUs to succeed the first generation. 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. 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
electric multiple units (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. This specification led to the development of the experimental DMU. 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. As such, the type had sufficiently demonstrated a promising reduction in maintenance costs was achievable, especially once initial teething problems were dealt with, as well as the wider value represented by a new generation of DMUs in the reduction of ongoing costs for BR. By 1983, experiences with the Class 210 had influenced planners to favour the procurement of a new generation of DMUs, but to also adopt a new set of specifications that were somewhat less demanding than the prior set. Specifically, it was decided to drop the top speed from 90 mph to 75 mph, as testing had revealed the higher speed did not deliver any perceivable improvement in journey times due to the typically short spacing of the stations the type was intended to serve. Furthermore, it was determined that a propulsion system delivering 7 hp per tonne would deliver sufficient acceleration. The requests for compatibility with other rolling stock were eliminated, although auto-coupling and auto-connecting functionality was added. In addition to a good ride quality, the specification included a sound level of 90
dB when at full speed, an operational range of 1,000 miles and an interval between major overhauls of five years or 350,000 miles. In comparison to the prior generation of DMUs, which typically used a pair of engines for each power car, the new generation DMU would only use a single engine per car; sufficient cooling was also provided that even with one failed engine, a two-car unit could continue to perform typical services without incurring a major performance deficient. 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. Initially formalised as a business specification, these requirements were transferred into a relatively broad technical specification that avoided any specifics other than those that were deemed essential for compatibility purposes. Thereafter, it was issued to various rolling stock manufacturers for a competitive tender. As a part of this process, these manufacturers submitted bids to construct an initial series of three-car prototypes as demonstration units. 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. In response to the specification, several submissions were received by BR. The bid submitted by BREL was heavily based on its successful 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. BR officials quickly opted to proceed with a pair of prototypes from both BREL and Metro-Cammell, issuing orders to these manufacturers henceforth. ==Prototypes==