I have quite a few cases on the layout where trains travel through walls. The biggest hole is just west of Proviso where the hole contains the mainline, staging tracks and a yard lead, seven tracks in all.
To hide the hole, I modeled the Manheim Road overpass right next to the wall. Helping the overall effect is a mirror and parking lot. The mirror is from Lowes. It is 20"X24" and cost $14. I had a local hardware store cut it to 20"X16" so that it fit the location better. In my visits to Proviso, I noticed a number of parking lots surrounding the yard that are used for storage of unused containers and trailers. I decided this would be a great location to add one of these lots.
The bridge itself is made from styrene sheet and Plastruct structural I beams. I laminated four styrene sheets to make the bridge supports. There are a bit odd looking but prototypical as you can see from the prototype picture.
For the parking lot I first used regular plaster to level out the area and then applied a coat of Woodland Scenics Smooth-It for the finished parking lot surface.
I still need to paint the bridge and weather the parking lot.