The British Rail Class 201 (or 6S) six-car diesel-electric multiple units (DEMUs) were built in 1957-1958 at Eastleigh and Ashford.The Southern Region Class 201-207 DEMUs are nicknamed 'Thumpers' due to the noise they made while in motion.These units were built with a narrow body profile to accommodate the restricted tunnels on the London-Hastings line, for which they were built.