British Railways 10100 was an unusual experimental diesel locomotive known informally as The Fell Diesel Locomotive (after Lt. Col. L. F. R. Fell, who was one of the designers). It was the joint production of Davey Paxman & Co, Shell Refining & Marketing Co and Lt-Col L. F. R. Fell, built for them by the London, Midland and Scottish Railway at Derby. Sir Harry Ricardo was also involved. By the time it emerged in 1950, nationalisation had taken place and it carried British Railways livery. The locomotive had six diesel engines, four of them used for traction. There were two auxiliary engines, both of which were AEC 6-cylinder units, and these drove the pressure-chargers for the main engines and the purpose of this arrangement was to enable the main engines to deliver very high torque at low crankshaft speed. Design The design for 10100, a collaboration between Fell Developments Ltd and H. G. Ivatt of the LMS, aimed to address several of the weaknesses perceived of diesel powered rail traction. Weight was reduced by using several small engines, meaning that both the engines and their supporting structure could be lighter. This was also expected to save time in maintenance as an individual diesel engine could be exchanged more easily and with lighter equipment. Transmission Using differential gearing to transmit the power, it was built as a 4-8-4 with the coupling rods connecting the centre four pairs of driving wheels. The coupling rods between the innermost axles were later removed, but as the four axles were driven by a single gearbox, it remained a 4-8-4. At it was the most powerful of BR's non-steam locomotives at the time. From 1951 it worked the expresses from Manchester to London, proving some 25% more powerful than 5XP 4-6-0s. While the mechanical transmission made it much lighter than the diesel-electric locomotives, its complicated mechanism made it difficult to maintain (a working model of the transmission is on display at the National Railway Museum, York). The locomotive had four main Paxman 12RPH 12-cylinder engines, each producing 500 brake horsepower at 1,500 rpm. Each engine was connected to the gearbox via a hydraulic coupling, which could be filled with oil to transmit power or drained to disconnect that engine from the transmission. The engine outputs were combined in pairs by two sets of differential gearing, and the output shafts from these two gearsets were then combined