AC units for Glasgow, however (classes 303 and 311) had power operated sliding doors, as had the Class 306 on the Great Eastern. In the Southern region, the early units were replaced in the 1950s and 1960s with new slam-door third-rail electric units, first of all the compartment commuter units 4-SUB and 4-EPB and later the much more comfortable longer distance trains which survived in use until the early years of the 21st century. These included the 4-CIG, 4-CEP and 4-VEPs; the former two being fairly similar while the latter was designed with more crowded seats and more doors to enable faster unloading and loading of passengers. Finally, also on the Southern division, new slam-door diesel multiple units were introduced in the late 1950s, these were classified as Class 201 and similar and were affectionately named 'Thumpers' due to the distinctive noise made by the engines. Demise The slam-door trains have had a long and robust service life, but were gradually replaced by newer units with automatic doors. These newer units are safer as the doors have central locking. This has now been fitted to surviving public rail line units. In the past, the doors on slam-door trains could be opened at any time, even while the train was moving. Sliding doors were first introduced on the London Underground in the 1920s, and by the 1930s, main line railway companies had already introduced a limited number of sliding door electric multiple unit classes, most notably the LMS and its Class 502 and Class 503 on Merseyside as early as 1938, and the LNER and its Class 306 on the Great Eastern Main Line and almost identical Class 506 on the Manchester to Glossop Line soon after World War II ended. The Scottish region saw its first main line sliding door trains in the 1960s with the introduction of the Class 303 and Class 311s. But it wasn't until the advent of the experimental Class 445 "PEP" stock in 1972 and its production derivatives such as the Class 313 AC/DC EMUs in the late 1970s that saw the beginning of the end for the slam door trains. The very similar 1980s-vintage Class 455 trains around London displaced the 4-EPBs while introduction of the 5-WES Class 442 Wessex Electrics in 1988 and electrification of the line to Weymouth saw the end of yet more slam-door trains. At the same time, BR had