Metropolitan Railway Bo Bo electrics

The Metropolitan Railway Bo-Bo Electric Locomotives were a fleet of 20 electric locomotives built by Vickers and Metropolitan-Vickers between 1922 and 1923 for the Metropolitan Railway's electrified main line from Baker Street to Verney Junction in Buckinghamshire, enabling the railway to work its Aylesbury and Verney Junction through-trains with electric haulage on the electrified inner section and then transfer to steam for the non-electrified outer section. The Metropolitan Railway was one of the pioneers of main-line suburban electrification in Britain, and these Bo-Bo locomotives were central to the system that served what the Metropolitan's marketing department was calling Metroland — the semi-rural suburbs of Middlesex and Buckinghamshire that the railway's tentacles reached.

The Metropolitan Railway had been electrifying its inner-London sections since 1905, using a 600 V DC conductor rail system, and by the 1920s the electrification extended to Harrow-on-the-Hill. Beyond Harrow the line continued as a steam-operated rural railway through the Chilterns to Aylesbury, Verney Junction, and the branches to Brill and Watford. The Bo-Bo electric locomotives were designed to work the through-trains that used both sections: hauling them electrically from Baker Street to Harrow, and then detaching to allow a steam locomotive to take over for the non-electrified section — or in some cases continuing under steam or diesel power where the locomotive could be switched between traction modes.

The 20 Bo-Bo electrics carried evocative names of historical and geographical significance — Benjamin Disraeli, William Ewart Gladstone, Lord Byron, John Milton, Oliver Cromwell, Sherlock Holmes, and others drawn from English history, literature, and notable figures associated with Middlesex and Buckinghamshire. When the Metropolitan Railway was absorbed into the London Passenger Transport Board in 1933, the Bo-Bos became part of the new organisation's fleet. One example, No. 12 Sarah Siddons, survived into preservation and is at the London Transport Museum Depot, Acton.