A slam-door train or slammer is a set of diesel multiple units (DMUs) or electric multiple units (EMUs) that were designed before the introduction of automatic doors on railway carriages in the United Kingdom and other countries, which feature manually operated doors. The name came about because of the characteristic noise made by the passengers slamming the doors closed when the train was about to depart. Some slam-door train designs featured doors that could only be opened from the outside, so passengers had to lean out of the window to reach the outside door handle. Slam-door trains had many more doors than newer trains (which tend to have only two sets of doors per coach); some designs featured a door for every individual seating bay. Some units had individual compartments, each with its own door and no access to any other part of the train; however, these were unpopular due to security concerns and the lack of access to toilets for longer journeys and many were later converted to standard corridor saloon design. The term "slam-door" could also refer to locomotive-hauled railway coaches that did not have automatic doors, but this usage is less common. The term "slam-door train" generally applied to Mark 1 and Mark 2 DMUs and EMUs. History Slam-door EMU and DMU trains were commonplace ever since the introduction of electrification. While there were early examples of the type, which are beyond the scope of this example, they became commonplace on the Southern Railway in the 1930s when it electrified its main routes around the south of London at 750 V DC (third rail), in particular, the line to Brighton. Slam-door diesel multiple units became common in the 1950s when British Railways (BR) sought to modernise its network and replace steam locomotives. Many one, two and three coach units were built for non-electrified lines all around the country, in particular, these were popular on branch lines where it was uneconomic to electrify. These units were later classified in the Class 101 – 129 series, dependent on the design. Meanwhile, large new fleets of electric slam-door trains were being built for use around the country. AC units were built for the newly electrified routes out of London Liverpool Street to Southend-on-Sea, Colchester and Clacton; also for some routes around Manchester. These AC units were later classified as Class 302 through to Class 312, again dependent on the design.