In connection with the construction of the Jubilee Line Extension project, fourteen diesel-hydraulic locomotives were purchased from Schöma of Germany to assist in equipping the tunnels prior to electrification. Weighing 33.88 tonnes each, they are 28 feet long and are powered by 500-horsepower six-cylinder inline diesel engines, which give them a maximum speed of 31 mph. They entered service from February 1996, are built to tube tunnel loading gauge, and were fitted with buckeye couplers when built. They are equipped with exhaust scrubbers to prevent soot and other particles from being expelled into the tunnels. The locomotives have since been used on other engineering projects. Numbers 1 and 5 have been modified by fitting Wedgelock couplers so that they can be used to shunt tube stock, whilst numbers 3 and 6 have been fitted with emergency couplers.

Several were rebuilt by Clayton to be battery electric locomotives, all 14 have subsequently been purchased by RSS at Wishaw. Following the success of the initial 14 locomotives, a few more have been imported into the UK too.