I am relatively new to model trains, using some HO inherited from Father In Law (Chessie retired). I am building a double tiered shelf system around the perimeter of an 18 x 14 room. On one wall which has an offset I would like to do a town scene complete with a bridge (already purchased) for the top rail. I would like the bottom rail to go subterranean like a subway and disappear (about 5 feet) and then reappear. I plan on running the two trains in opposite directions. How do I calculate the up and down trestle/bridge supports needed?  Is there any power issues on doing so? Is there any power issues on such a long fun of track.  I have a couple of large sized controllers.  I have about 12 inches of clearance to work in but want to use maybe 2 or 3 inches of that for a river (cross cut view) under the top bridge.