A hole in the wall X4 - a liftout bridge (page 12)

[nachoman]

One potential problem - suppose you are running a train and as it goes into the next room it derails. I suppose it isn't much worse than a tunnel, but if the tunnel is in the same room you can usually hear it derail. Have the train derail in your living room, and not notice, and you could be sending a whole train to it's demise on the tile below.