Can multiple trains use a bridge or tunnel at the same time?
Is there a setting to permit multiple trains in a tunnel or across a bridge at one time? I can't seem to find anything related to it, yet I would imagine with more complicated layouts this kind of setting would become essential.
If it is not possible, what are some tips/tricks to maximize crowded traffic through longer tunnels or bridges?
There are two relatively simple tricks you can use here:
Split the bridge/tunnel in twain, by digging a hole in the middle of the mountain or building a small island in the middle of the bay; big enough to place signals on.
Double (or triple) up on the bridge/tunnel in both directions.
For best results, combine both approaches, but do watch the money meter: both of these things can be pretty expensive.