class-121-bubble

The BR Class 121 'Bubblecar' was a series of 16 single-unit diesel multiple unit vehicles built by Pressed Steel of Oxford in 1960 for British Railways, designed for the most lightly-trafficked branch lines where even a two-car DMU formation would be uneconomically large and a single self-propelled railcar could adequately serve the sparse passenger demand. The Bubblecar nickname — applied by railway staff and enthusiasts to both the Class 121 and the related Class 122 — referred to the distinctive profile of the single unit with its large cab windows wrapping around the front, giving the vehicle a wide-eyed appearance that became affectionately familiar on the rural branch lines of the Western and London Midland Regions.

The single-unit concept was the most economical possible approach to branch line operation: one vehicle with one driver, needing no locomotive at all, capable of carrying perhaps 50–70 passengers in reasonable comfort on the rural services that justified the branch's continued existence. For lines like the Cholsey and Wallingford branch, the Kemble–Cirencester branch, and numerous other WR rural services that survived into the 1960s, the Bubblecar was the difference between keeping the line open and closure. The Class 121 could also be coupled to other DMU vehicles for peak-hour strengthening or through workings onto main-line services.

Three Class 121 vehicles are preserved: 55020, 55024, and 55028, several of which have continued to operate on heritage railways and even on Network Rail for departmental purposes.