You pay for someone (another rail company operating at that railyard) to move it for you into storage, where you pay the storage costs. If you need that railcar somewhere else, you pay someone to transport it somewhere else, same as how cargo rail would work.
The thing I haven’t read in this thread: how do you get your personal trail car to the station, and how do you move it about on your destination?
It sounds cool but very inflexible.
You pay for someone (another rail company operating at that railyard) to move it for you into storage, where you pay the storage costs. If you need that railcar somewhere else, you pay someone to transport it somewhere else, same as how cargo rail would work.