The British Rail Class 201 (or 6S) six-car diesel-electric multiple units (DEMUs) were built in 1957–1958 at Eastleigh and underframes were built at Ashford. They were usually in the formation: DMBSO-TSOL-TSOL-TFK-TSOL-DMBSO.
The Southern Region Class 201-207 DEMUs are nicknamed 'Thumpers' due to the noise they made while in motion, owing to the four-cylinder engines.
These units were built for the London-Hastings line, with a narrow body profile to accommodate the restricted tunnel loading gauge on that line.
