When the first experiences of the British Transport Commission's 1957 Modernisation Programme, intended to replace the ageing fleet of steam locomotives with diesel and electric traction,
were evaluated it became evident that a need remained for a large class of lighter, more powerful, diesel locomtives at the top end of the BR Type 4 range (2,000bhp-2,750bhp).
The performance of the English Electric (Class 40) Type 4's and British Railways (the "Peaks", later classes 44, 45, and 46) Type 4's had not attained the results expected of them.
Not only was the power to weight ratio of these designs regarded as being too low, there was some evidence to show that the 1Co-Co1 wheel arrangement, designed to spread the 130 ton (plus) weight over 8 axles in
order to minimize wear on the rails, was having the opposite effect.
The Western Region, influenced by German dieselisation experiences, had gone its own way with lightweight diesel hydraulic locomotives using, initially, the AIA-AIA wheel arrangement on the D6XX (Class 41 "Warships"), and later B-B (the Class 42/43 "Warships"), and C-C (the Class 52 "Westerns") wheel arrangements.
The "Westerns" with a power output of 2,700bhp weighed in at just 108 tons, compared with the Class 45 "Peaks" weighing 138 tons with a power output of 2,500bhp.
However, BR management were far from convinced that the future of diesel traction lay down the hydraulic transmission path, and numerous problems with the reliability and availability of the diesel hydraulics with their German designed engines and transmissions
had done nothing to impress them to the contrary.
Whilst BR procrastinated on the dilemma three private locomotive builders took up the challenge and began to design prototype lightweight Type 4 locomotives, all using electric traction motors driving a six axle Co-Co wheel arrangement, with a targetted weight distribution of no more than 19 tons per axle.
In 1962 English Electric produced their highly successful DP2 prototype which, although externally resembling a Class 55 "Deltic", became the forerunner of the Class 50's introduced in 1967.
DP2, with its new English Electric 16 cylinder 16CSVT engine, weighed just 105 tons and had a power output of 2,700bhp.

Also during 1962, a consortium consisting of the Birmingham Railway Carriage and Wagon Company (BRC&W;), AEI, and Swiss engine manufacturer Sulzer, produced "LION" using an uprated version of the tried and trusted Sulzer 12LDA28-B
power unit that was installed in the Class 45's and 46's. The locomotive weighed 114 tons and the new 12LDA28-C unit produced 2,750bhp.

The Brush Electrical Engineering Company however had been the first of the private builders to get off the mark and had, in 1961, introduced their "FALCON" prototype. Using twin Maybach MD655 power units (as installed in the Class 52 "Westerns") producing 2,880bhp (later reduced to 2,700hp),
and modified Brush Type 2 (Class 31) bogies; "FALCON" weighed in at 115 tons.
"FALCON" and "LION" are often regarded as the prototypes that eventually led to the Class 47's, but this is something of an exaggeration. The urgency of the British Transport Commission to introduce a large class of lightweight Type 4 locomotives
to replace steam traction by their self-imposed deadline of 1968, resulted in full production of the Brush Type 4's commencing before the performance of the prototypes had been fully analysed and evaluated. In fact construction of the Brush pilot
order of 20 Type 4's (D1500-D1519) had begun even before "LION" had turned a wheel!
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