A slam-door train are diesel multiple units (DMUs) or electric multiple units (EMUs) that have doors that can be opened and closed by passengers, these trains were made before automatic door trains were made.
They're called slam-door trains because of the noise they made by the passengers slamming the doors shut when the train was about to leave.
Some slam-door trains (such as the British Rail Mark 3) had doors that could only be opened from the outside, so passengers had to lean out of the window to reach the outside door handle.
In 2005, all slam-door trains were replaced with trains with automatic operated doors.[1][2] The last slam-door train was withdrawn by South Eastern Trains in December 2005.[3]