Powered by a Gardner 8L3 diesel engine rated at 204 bhp with drive through a fluid flywheel, the shunter uses a mechanical transmission system with the wheels connected by coupling rods to the driving shaft thereby ensuring maximum adhesion available from the six wheels. A five-speed gearbox is fitted with a separate reversing gearbox which means all gears may be used in either direction of travel, to achieve a maximum of 28½ mph. Only one cab is provided, which stands out above the low bonnet covering the engine and equipment. The bonnet has a chimney at the front, somewhat reminiscent of a stovepipe chimney on a steam engine! These locomotives are very similar to the later class 04 diesel shunter.

The Class 03 outlived all British Rail's other small diesel mechanical shunters, lasting until 1989, and now live on principally on preserved railways and with the MoD, although one, N°03179, was reinstated into traffic by WAGN, repainted in their livery and used for shunting EMUs at Hornsey.