The BR Standard coach, later known as the 'Mark 1', was based on a separate underframe with stiffening trussing which carried a steel body superstructure. The underframe came in two lengths, 57ft and 63ft.
None of the DMUs built by BR or its contractors used this design as it was impossible to arrange the engines, gearboxes, cardan shafts, batteries and fuel tanks in the spaces left by the trussing.
There were two design 'Centres of Excellence' for the diesel railcars: Derby was responsible for the branch line and suburban sets and Swindon for the later Inter-City and Cross-Country units. The first of these to be designed were the units for lightly trafficked lines. The specification issued by the Railway Executive in 1952 for self-propelled diesel railcars for these services called for a power-to-weight ratio which, because of the limited power available from contemporary diesel engines, could only be achieved using aluminium alloy construction. Using these design principals, from the 'Derby lightweights' onwards the underfloor area was essentially 'clean' and the body strength was supplied by the size and arrangements of the longitudinal members, in later models assisted by the body. These DMUs had nothing in common with the BR Standard coach with its steel construction and trussed underframe.
For the later Inter-City and Cross-Country railcars the specification was changed to be comparable with the Standard coaches: end load resistance of 200 tons, Pullman gangways and buckeye couplers. Swindon designed these railcars using an integral monocoque construction (no separate underframe) and the first went into service on the Glasgow - Edinburgh route in 1957. The only common factor with the standard coach was the use of steel.
The point about these Swindon DMUs, and the later Trans-Pennine sets, was that their basic design was adopted for BR's Mark 2 stock.
