You would have to go by the track leading up to and after you cross the bridge (see Robert Anderson’s tip about track gauge). The track across the bridge should be on the same level as the ground track, so for the piers you might have to do some measuring to get it right. The piers should support the bridge on either side without buckling the track on entering or exiting the bridge.

The height of the cutout that the bridge crosses, whether it be water or another train, should be in accordance with the bridge piers. For water, I usually leave about a half inch (at least) of shore line for N-Scale, so the water doesn’t seem like it’s overflowing its banks. If passing over other trains, put the lowest part of the bridge about 2-1/2 inches or even three inches above the tops of the lower train so an “accident” don’t occur. Make all of these measurements on the track rail height.

I hope this post helps. Remember to test-fit everything before nailing or gluing it down.

John