There is a good bridge thread active today. (My copy/paste is having fits)
The biggest thing to remember would be the height of the trains you run on the bottom level if it crosses over. My worries were raised pantographs, loaded Evans Autoloader and a raised rocket launcher car.(7inch for me{height+track} So to make the 5"-ish PW trestles taller, 2x4 painted as cement. Putting a beveled crown on top of each for shedding rain water for realism)
I would for sure construct support or use a whole lot of tressel piers on any brand track if elevated.
On a shelf you want the train as visible as possible. Being right on the edge of a shelf helps. Clear support, by plastic, glass, or stringers("rods", etc. Run parallel to rails), will ensure it is seen best,and allow a wider shelf to catch a derailed train to be used. With GG's track, the many ties will help hide the undercarriage some too. Two tracks? Switches increase chance of derail and the line close to the wall will need to be elevated more to be seen well.
I would have went narrow shelf or plastic, but the shelves were already there.
Find a place nearby for the ladder or folding steps too