The cars that are not in use are still on the layout, and form a train that sits on a hidden staging yard track that is not used during ops sessions. About half of those cars are ones that I should probably sell off or give away as I no longer want to run them on the layout (for one reason or another). The rest get swapped out from time to time with other cars, just to change things up a bit.
This train has now grown to 27 cars, and is the longest train on the layout. I can, and have, run longer trains, but for ops sessions my freight trains are limited to 2 locos, 21 cars and a caboose, as that's all that can fit on my arrival/departure tracks without interfering with other tracks.
Here's a video of this train running southbound through Davis: