A func_train that starts at one point, gets toggled on, travels to a second point, but the second point doesn't point the train to a third point to continue so it just stops. Though this would only work for a sliding door as there is no way to change the func_train angle at each point.
The absurdly long wait time is probably the best solution really, as long as the game doesn't last an absurdly long time

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I thought of a few other ideas, but the entities involved aren't targetable, so it won't work. I think it would be nice to see some new entities and new features for existing entities in the next version of Tremulous.