The British Rail Class 42 Warship diesel-hydraulic locomotives were introduced in 1958. It was apparent at that time that the largest centre of expertise on diesel-hydraulic locomotives was in West Germany. The Western Region of British Railways negotiated a licence with German manufacturers to scale down the German Federal Railway's "V200" design to suit the smaller loading gauge of the British network, and to allow British manufacturers to construct the new locomotives. The resultant design bears a close resemblance, both cosmetically and in the engineering employed, to the original V200 design. Warship locomotives were divided into two batches: those built at BR's Swindon works were numbered in the series D800-D832 and D866-D870, had a maximum tractive effort of 52,400 pounds-force (233,000 N) and eventually became British Rail Class 42. 33 others, D833–D865, were constructed by the North British Locomotive Company and became British Rail Class 43. They were allocated to Bristol Bath Road, Plymouth Laira, Newton Abbot and Old Oak Common.

The Western Region was a region of British Railways from 1948. The region ceased to be an operating unit in its own right on completion of the "Organising for Quality" initiative on 6 April 1992. The Region consisted principally of ex-Great Western Railway lines, minus certain lines west of Birmingham, which were transferred to the London Midland Region in 1963 and with the addition of all former Southern Railway routes west of Exeter, which were subsequently rationalised.