British Rail assigned Class 52 to the class of 74 large Type 4 diesel-hydraulic locomotives built for the Western Region of British Railways between 1961 and 1964. All were given two-word names, the first word being Western; thus the type became known as Westerns. The Westerns, like the Warships, were numbered in BR's original diesel serial number system, before the TOPS class-plus-number system took over: Warships were in the D800 range, and the 74 Westerns were numbered from D1000 to D1073, though on TOPS itself the Westerns' new numbering was used—the loco number by itself (e.g. 1023) appeared on the system.The locos themselves never had the new numbering applied to their bodywork either.