As originally built, the traction arrangement of the Class 142 consisted of a Leyland TL11 200 HP engine, a Self-Changing Gears mechanical automatic gearbox and a Gmeinder final drive unit on each car driving only a single axle. This propulsion arrangement was in part taken from the Leyland National bus, as well as shared with the earlier Class 141. Unlike the Class 141, a microprocessor-based controller for the automatic transmission was used from the onset, allowing the reliability issues posed by defective relay logic and poor earthing present on the predecessor to be entirely avoided. Another improvement was the installation of auto-couplers and auto-connectors that enabled the Class 142 to work in multiple with the Class 150 Sprinter DMUs. The Class 142 is fitted with double-folding external doors, identical to the preceding Class 141. Each car has a fuel capacity of 125 gallons.