Do you mean storing cars at the request of the owning railroad? Normally any railroad wants to get rid of any foreign cars on its lines if it doesn't have a prospective load for them, since it's paying "per diem" (per hour instead of per day for some years now) to the owning road until it can interchange the cars back to the owner. There have been cases under past ICC regulation where a small shortline had more cars than it could physically accommodate if they all returned home when business was way down, but today, as far as I know if a railroad has many surplus cars it will find someplace to store them on its own rails--even in some cases to the extent of clogging up passing sidings to the detriment of revenue traffic.