Google 'Storage in Transit' aka SIT car.
If the cars are empty, that would be more accurately described as just plain old "S"
To BandA's question, it's not uncommon for owners or lessees of private equipment to lease track from a railroad to stage or hold surplus equipment that won't fit within a loading facility. Leases vary in scope (meaning number of track-feet included), price and term (from, say, one month to multi-year). The track leased could be a specific track or a "virtual" track, meaning the lessee is allotted X track-feet in Poncha Springs. It may even be written that the track leased is track 3 in the Poncha Springs yard, but the yardmaster (or whomever) is open to place the cars wherever they fit best within the terminal. In my experience, this is provided (sometimes reluctantly) as an assessorial service by a carrier without commercial link to loaded moves.
Receivers will also lease trackage to hold excess inventory outside their gate. Doing so allows the receiver to avoid punitive demurrage charges associated with a railroad holding private cars in a yard on constructive placement. I guess you could argue that this would technically storage-in-transit, but SIT in its purest sense is the staging of product beyond its loading point in anticipation of an order.