The BR class 40 locomotives were among the earliest production mainline diesel locomotives to enter service, being derived directly from the final Southern Railway experimental locomotive, number 10203. Fitted with the highly reliable English Electric 16SVT engine set at 2000bhp inside a conventionally built bodyshell and riding on heavy 4-axle bogies BR called these 'type 4' locomotives, intended to undertake the heaviest of duties. Deployed on West Coast express passenger services the massive weight of these early diesels made them appear poor replacements for the fast and powerful Stanier pacifics, but as electrification progressed their robust construction made them superb heavy freight engines. Sometimes nicknamed 'Big D' for their impressive bulk and power the 40s were later referred to as 'whistlers' from the whistling sounds from the single-stage turbo chargers, a trait shared with class 20s.
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