Train levels are, as the name implies, levels that take place on some form of train.
Depending on the setting, a train level can take place on a classic steam powered locomotive or it can be set on a futuristic high tech, sci-fi stylized train.
Depending on the game, the train may be a mere backdrop (as in Fatal Fury 2
) or it can be an integral part of the core level design (as in Blood
Finally, the train does not always having to be moving, though more often than not a moving train is used to bridge two levels together, to show off the developer's engine, or to simply create a heightened sense of excitement in the player.