Thanks for all the help. So far the compressed air sounds the best of all.
I would not need to stop all cars, so the paint brush idea (even though it sounds neat) would not work because it would then slow down every car.
The testing I've done shows me that a light car will run the 20' along with the 2" hump, and at the end it will just touch the bumper I put at the end of the track.
I've built the level for the freight yard 2" higher at one end of the 20'. You don't notice it, but that should, along with the hump, put the lighter cars at the end of the 20'.
It's the heavy cars I might need to slow down some.