I have two trains of hoppers. Both have nearly identical cars, one with coal (plastic) loads and the other empties. On one side of the layout (about 14 feet from the other) is a coal mine. On the other side of the layout is a 1950's era coal-fired electric generating plant. The coal mine is a combination underground and open pit, with a tunnel entrance from which the loaded cars appear, running underneath loading towers. A pair of GP7's pull the loaded cars out to the main yard, where they can be picked up by "road" power, either multiple F-units or a 2-10-4, or a pair of double-heading 2-10-2's (depending on the whim of the dispatcher...me). The loaded hoppers are pulled up a 4 percent grade and around the long wall, to the Denver yard, where the waycar (caboose) is uncoupled, with a flagman standing by down the track. The hoppers are then pushed in behind the electric plant to be "unloaded". Only, they don't get unloaded. You see, prior to the loaded hoppers being pushed onto the siding, the road switchers down at the mine have pushed the string of empties up a hidden track (at least it is partially hidden) so they appear from behind the electric plant. The motive power, having uncoupled the caboose, backs the loaded hoppers until they can couple to the empties. It then pulls them out onto the main, couples them to the waycar, and then uncouples the loaded ones from the empties, and backs the loaded cars onto the track behind the power plant. The engine then uncouples from the full cars, couples onto the empties, and proceeds around the return curve going eastbound with the empties. Meanwhile the two road switchers go through the tunnel to pick up the loaded hoppers and return them downhill through the tunnel to the mine entrance. The empties will be returned to the vicinity of the mine, stored on a siding until the loaded cars are switched out to the yard lead. This can be continued as long as you like.
Likewise, although this is done in plain sight, several stock cars can be "loaded" at a cattle pen and transported a shorter distance to an Oscar Meyer packing plant. A few hoppers also are reserved for a siding near a farm house, where a farmer is unloading sugar beets. These hoppers (right now, just two...that's all the siding at the Great Western Sugar processing plant can handle...are assembled with whatever other cars make up a turn. Empties from the plant are returned to the rural siding. Etc.