The British Rail Class 151 was a prototype class of diesel multiple unit (DMU) developed and constructed by the British railway equipment manufacturer Metro Cammell. It was designed primarily to serve as a successor to the earlier first-generation "Heritage" DMUs operated by the British Rail (BR).
Development of the Class 151 commenced during 1983 in response to a specification issued by BR calling for a new generation of DMU with which to equip its fleet. Both Metro Cammell and British Rail Engineering Limited (BREL) were selected to produce prototypes of their design submissions. Accordingly, a pair of 3-car units were constructed, which entered trial service with BR during 1985. If it had emerged successful, the Class 151 would have likely become the basis of Sprinter family of DMUs that were produced during the 1980s. However, following a competitive evaluation against the rival Class 150, the type did not secure a production contract, having lost out to its BREL-built rival. Both units were withdrawn during 1989 and, despite multiple vendors planning to restore them for service, they were ultimately scrapped.
Background
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, 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. 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 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 British Rail Class 210 diesel-electric multiple unit. 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. 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 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 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.