File:150hp Winton rail motor for Manly - Cleveland line, Cleveland, Redland, December 1930.jpg

In the 1930's, many European and American railways were pursuing lightweight high-speed diesel trains and NSWGR followed suit with a similar train to provide main line services on the Broken Hill line in 1937. The Silver City Comet, as the train became known, consisted of a locomotive, called a 100 Class Power Van, and a number of passenger trailers. The power van was of all-steel construction and was powered by two British-made Harland and Wolff "Harlandic" 330 hp 8-cylinder 2-stroke diesel engines driving through Voith-Sinclair hydraulic transmissions. The engines were built under licence from Danish shipbuilding company, Burmeister and Wain, by Harland and Wolff at their Belfast, Northern Ireland works. These engines were not as powerful as required for a fully loaded Comet set and a multi-valve cylinder head was designed but was not supplied from Belfast until 1943 due to production difficulties during World War II. Two smaller National 32-hp 4-cylinder diesel engines were also fitted to generate power for the air-conditioning and to drive auxiliaries.

In parallel with the Comet development, a smaller branch line rail motor was also designed and built by Eveleigh Workshops, the first entering service in 1938. The 400 Class were of composite steel and timber construction and were fitted with a repeat of the twin 150 hp Leyland E47/1 petrol engine and Lysholm-Smith transmission used in No.38. The 400 class were double ended with a small passenger compartment and could haul up to two 500 Class passenger trailers.

During the mid-1930's a number of different engines were trialled in the CPH Class. These were a British 150 hp 6-cylinder AEC petrol engine (1933), a US-made 185 hp 6-cylinder Winton petrol engine (1934) and a 120 hp 6-cylinder Leyland diesel engine (1939). Some of these units remained in service for a considerable number of years up to the end of World War II.