It was also intended that the DMU could be assembled akin to building blocks, comprising between two and four cars that could 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. Under this process, responding manufacturers submitted bids to construct an initial series of three-car demonstration units. A 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 compelled manufacturers to lean towards existing industrial practices for their submissions. 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. The railway engineering company Metro-Cammell also bid, offered its own design that employed rivetted aluminium construction; this approach produced a meaningful weight reduction over conventional methods. BR officials opted to proceed with a pair of prototypes from both BREL and Metro-Cammell, resulting in the Class 150 and Class 151 respectively. Between 1984 and 1985, these prototype units were delivered to BR and commenced their trial service period. Both the Class 150 and 151 units were subject to extensive evaluations with the aim of a larger production order being forthcoming for the more successful of the two types. Testing revealed that the Class 150 had exceptional ride quality, as well as fully satisfying the 50percent engine-out performance requirements. It was also determined that both types achieved adequate noise levels for its intended service sector, but were also noted that this area would likely pose an issue if they were ever directed towards the higher end of the market. Early concerns regarding the body bending frequency of the Class 151 did not prove to have major substance to them. Ultimately, the Class 150 prototypes proved to be more reliable and an order for 50 two-car units was accordingly issued to BREL, leading to a production batch of Class 150s. Even prior to the introduction of the Class 150, there was a recognised interest within BR at potentially tasking the new DMU with the replacement of other services,