D0260, named Lion, was a prototype Type 4 mainline diesel-electric locomotive built in 1962 by a consortium of Birmingham Railway Carriage and Wagon Company, Sulzer the engine maker and Associated Electrical Industries, at BRCW's Smethwick works near Birmingham. The locomotive's number was derived from its works number, DEL260. Specification Lion was a private venture to meet a requirement from British Railways for a powerful locomotive of Co-Co wheel arrangement, as a substitute for the earlier 1Co-Co1 locomotives such as the Peak classes. The specifications were revealed by the British Transport Commission (BTC) at a meeting on 15 January 1960. Train heating was to be by both steam and electric train heating (ETH). Unlike the earlier Pilot Scheme, the BTC expected that these prototypes would be funded by the makers, rather than bulk orders being placed sight-unseen. For the Type 4, that gave rise to three prototypes: Falcon, DP2 and Lion, eventually leading to the Class 47 and Class 50. Lion used a more powerful development of the same engine as the Class 44 Peaks, but lighter overall and with a more reliable bogie design. It was of Co-Co wheel arrangement and was fitted with a twin-bank Sulzer 12LDA28C engine of . It was a development of the 12LDA28A used for the Peaks. At that time, the Sulzer engines were favoured over English Electric's V12 option, despite their greater cost of £45,000 vs. £26,000, and their weight of 22.3 tons vs 19.4 tons. The locomotive had a design maximum speed of and weight of . The design dimensions of both Lion and the Class 47 were the same, although the Class 47 ended up a few tons overweight. Construction Body Lion's overall construction was as a truss from bufferbeam to bufferbeam, using the full height of the body to cant rail level as the structure. It was built from welded thin steel sheet, as either sheet or pressings. This gave a structure which was stiff and strong, yet avoided the weight of heavy underfloor girder sections. Truss structures had been used previously for the Peaks, although with the channel underframe as well, but these had been the diagonally-braced Pratt truss, which limited access through the sides and made access for maintenance difficult, often requiring an overhead crane. Lion used the uncommon Vierendeel truss, which is characterised by its rectangular openings. Although an inefficient structure for bridges, owing to the restricted bracing,