Unless you are modeling real heavyweights like the D&RG the typical ON30 consist is much smaller and lighter. Most S scale bridges and tunnel portals are adequate for the smaller size locos and cars,not overwhelming the rolling stock.
If I ever get my new layout started I will use S scale wooden bridge, trestle and tunnel portal/retaining walls.
Go to a well stocked hobby shop or a scale model RR show and examine some of the built up display models. They fit in extremely well.
Consider that a bridge designed for a Big Boy looks rather silly with an O-4-O crossing it pulling a bobber caboose!
Tony