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.