11001 was one of the first British Railways diesel locomotives. It was built in 1949 at British Railways' Ashford Works. It was designed by O. V. S. Bulleid when he was Chief Mechanical Engineer of the Southern Railway. It was powered by a Paxman RPH Series 1 engine, capable of delivering 500 bhp at 1,250 rpm. It was driven via a Vulcan Sinclair fluid coupling to an SSS (Synchro-Self-Shifting) Powerflow gearbox. The gearbox provided three forward and reverse gears in either high or low range. It gave a top speed ranging from 5 mph (8 km/h) in 1st gear, low range up to 36 mph (58 km/h). It had an 0-6-0 wheel formation.