Jamie, a keen member of the online model train club sent in this tip:

“I use those little tubes available for garden sprinkler systems as sewerage and drainage pipes. I cut them into short pipes lengths of 1″, 1 1/2″ or even up to 2″. I then paint them a steel or concrete color. For culverts, I bought a pack of those bendy drinking straws the kids love to use. Because they bend they work well as corrugated parts on some culverts. Obviously I paint those too.”