The British Railways Mark 1 SK was the most numerous carriage design ever built in the United Kingdom.

There were two variants, those built for the Midland, Scottish, and Eastern / North Eastern regions had six seats per compartment, with fold-up arm-rests which folded into the seat-back, while those built for the Southern and Western regions, with their heavy commuter loadings into London, had eight seats in each compartment, and no arm-rests.

Over time, some SKs and BSKs were re-allocated from other regions to the Western, who tended to try to stitch the arm-rests in the up position.

In 1956 the standard livery changed to maroon with black and gold lining, except for the Southern region stock which adopted an unlined dark malachite green.

In the 1980s the NSE livery of white with blue at window level and red stripe under was introduced for stock in the south east of England.