I've actually got a half-finished map that works like this (The map was originally planned just as a demonstration of how it works). I haven't encountered any problems with the size/details of the func_trains (I used 8 pieces, 3072x832qu each, albeit with low details). I think the main problem with the detail is to keep it low enough to get a decent performance.
There is also a problem with players nocliping outside the train, which blocks the moving pieces and make them clip into eachother. I have been trying to work around this by only using noclip-brushes, but I never finished my test.
I can give you the .map-file if you want.
EDIT: Oh, I realise you mean having a single huge func_train. My system has 8 pieces that "rotate" around the train.