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 and 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 concept was relatively ambitious, calling for a maximum speed of , acceleration comparable 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 a failed unit, and to comprise either a three- or four-car consist. This specification led to the experimental British Rail Class 210 DMU. However, it was found that relatively expensive equipment was needed for the performance specified, particularly to provide sufficient speed, acceleration, and through-passenger access; maintainability also suffered due to space limitations. 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. By 1983, experiences with the Class 210 had influenced BR planners to favour procuring a new generation of DMUs, but to also adopt a new specification that were somewhat less demanding than before. Specifically, it was decided to drop the top speed from , 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. Furthermore, it was determined that a propulsion system delivering 7hp 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 , and an interval between major overhauls of five years or . While the prior generation of DMUs typically used a pair of engines for each power car, the specification called for only a single engine per car, as well as for sufficient cooling so that, even with one failed engine, a two-car unit could maintain a typical service performance without major deficient.