Here's a cute trick for complicating model railroad operations. Last year, James McNab saw a tank car appear on the Iowa Interstate, the short line railroad he models. The tank car sat around for three days, then disappeared. James figured out it was a misrouted car that wasn't supposed to come to the Iowa Interstate. It sat around for a couple days, getting in the way, until folks could figure out where to send it.
James thought this was a nice way to complicate operations on his model railroad of the same track. He now occasionally places a misrouted tank car on the layout, and warns crews either to "leave it alone" or bring it back to the yard. It's a quick and easy way to complicate operations. For folks modeling the 1970's (or perhaps even the 1930's), a siding full of idle freight cars might be another way to increase the puzzle challenge of operations.