Culverts - Model Railroader Magazine

I'd suggest looking at as many prototype photos as you can find.  If there's one that looks like the area in question on your layout you'll have something to work from.

I have a number of culverts on my layout, some in finished scenes, others not.  Maybe these will supply some ideas.

This concrete box culvert was built from styrene sheet and Evergreen C channel.

The culvert at right under the track was made from soda straws, while the one under the dirt road used the corrugated portions of other straws (the smooth parts were buried).

This one has home-made plaster castings of WP prototypes, but could also have used commercial parts or have been scratched from styrene.

This is a larger version of the plaster castings used above.  It's actually see-through if you get low enough, and has a styrene box separating the castings on either side.