The D600 Warships were a class of just five locomotives, but of which the pioneer, No. D600, was the first British mainline diesel-hydraulic locomotive and the first Type 4 diesel completed for BR when it was delivered in late-1957. Ordered for British Railways' Western Region by the British Transport Commission (BTC), all five of these Type 4 A1A-A1A diesel-hydraulic locomotives were built by North British Locomotive Company in Glasgow. As part of the BTC#s Modernisation Plan, various diesel locomotives were trialled and whilst the majority were diesel-electric, the Western Region (WR) was adamant about its preference for diesel-hydraulics.

Whilst the WR was concentrating its efforts on designing a smaller Type 4 machine based on the German V200s - which would become the D800s (Class 42 and 43) - the BTC pressed on with its plans for the D600s with which it sought to provide a comparison with the Type 4 diesel-electric D200s (later Class 40). The D800s - which too were known as Warships - would go on to be the more numerous and more successful of the Warship types. Employing a B-B wheel formation, the D800s were much lighter than the D600s, whilst offering a comparable or higher power output and tractive effort.

The first two D600s, Nos. D600 & D601, were allocated to Swindon but by June 1958 both had moved to Plymouth Laira where there were joined by the final three locomotives. Named after Royal Navy vessels, the D600s carried nameplates bearing the subtitle #Warship Class#, a feature shared with the D800s. On 16th June 1958 No. D601 found fame as the first diesel locomotive to haul the Cornish Riviera Express non-stop from Paddington to Plymouth and the class remained in use on fast Bristol/West of England services until they were displaced to secondary duties following the widespread introduction of the D800s.

As built the locomotives carried disc headcodes but these were later changed to a pair of two-character headcode boxes mounted either side of the nose. As the 1960s progressed and the BTC sought to standardise the locomotive fleet, the D600s were an obvious candidate for early withdrawal, being a class of just five that had been well and truly outshone by their smaller, lighter D800 counterparts. All five were removed from traffic by the end of 1967 and although allocated Class 41 under TOPS, all had been sold for scrap long before the system was implemented.

- Split Headcode Boxes
- Revised Bodyside Grilles
- Equipped with a Next18 DCC Decoder Socket
- Length 235mm (over couplings)