Entering service between January 1958 and January 1959, the class initially worked on the London-Plymouth-Penzance route of the Western Region. On 16 June 1958 D601 hauled the Cornish Riviera Express non-stop from Paddington to Plymouth—the first diesel locomotive to do so. The maximum permitted loads for a D600 on such a run were 375 tons (381 tonnes) westbound (climbing the 1-in-37 of Dainton Bank, west of Newton Abbot, and up Hemerdon Bank's 1-in-42 in the opposite direction). The D600s continued on the fast Bristol/West of England trains until a dozen D800 Warships had been accepted into service. Later they were largely restricted to the line west of Plymouth, finally being withdrawn "" in December 1967. They were noted for being capable of over if worked well and did run at 100 mph (160 km/h) during their very early careers. D603 was damaged in an accident and was returned to NBL for repair in 1960: the cast light alloy cabs were replaced with sheet steel as the original NBL subcontractor for these items was not prepared to fabricate a small, one-off order. Swindon had a spare cab which was not used and survived long after the locos had been withdrawn before finally being sold for scrap.