Hump Yard
I don't have any idea where working car retarderds can be found, but there may be one ? I am presently making Altoona; 17+ freight yards. A hump can be created verry simply buy raising the hump track a few feet. And at the end of the hump yard simply raise the tracks slightly again, so as to prevent run-throughs. The cars will slow and roll back slowly. Experiment with sevearal track elevations to get the desired speeds. My freight yards are going to be 5 miles long with 5 hump yards, and some places 30 tracks wide. I'm using a low poly track like MP wood and MP Rusty. PS: I used to work in a hump yard in Camden NJ.