IME most railway staff live in a fantasy world where heavy wheeled objects become easier to load onto a train when folded rather than wheeled, which is why the attitude to dismantleable tandems seems strange.
Of course, in reality it rather depends on where and when you do the dismantling, how much other luggage you're also trying to carry, and whether you're going to have to carry it the length of a crowded platform after the train arrives because you don't know which end the bike space will be at.
It may be that they don't want you blocking the vestibule while you get the spanners out...