As you know, when a player steps aboard a func_train, the player will be attached to its axial vector (or something like that!), so when the train starts moving, so does the player, or when the train rotates 90 degrees, so does the player, etc. 

There is a flaw in this behavior. The corpses lying in the func_train's path will attach to the train as well, so when the train reaches the corpse, it stops for a brief moment, even though the train is entirely made of nonsolid surfaces. 

Noclip mode hasn't been taken into account, either. And there shouldn't be any reason to attach a player in noclip mode to a func_train. This is in fact the exact opposite of what you expect to happen in noclip mode, as all collision detection should be disabled. If you can walk through walls, why a train can't go through you?