Colorado
Union Tack Car was founded to supply Standard Oil with tank cars. After the split up of Standard they leased cars to others. UTLX tank cars can be found almost anywhere.
Other car builders lease cars as well. The leasing arm of American Car and Foundry is Shippers Car Line. SHPX tank cars are everywhere too and have been for decades.
For modeling the 1930s and 40s oil industry the best O scale cars that we have are the Atlas, Lionel and MTH Railking 8,000 gal tank cars.
Here is the Atlas 8,000 gal tank car in SHPX. It is product number 8660. Cars in basic black are very common in the real world but the models tend not to sell as well as more colorful cars. Grab black leaser cars when you can find them!
MTH made a diecast railking tank car in basic black for Shell some years back. A scale oriented friend has one and it fits in nicely with his steam era Atlas cars.
While 10,000, 12,000 and 16,000 gallon tank cars were used by the oil industry we only have them available in O as kits or in brass. As you go back in time the more likely you are to find a car used for a commodity where it did not have sufficient volume to be loaded to maximum weight capacity. The Atlas 11,000 gallon tank car represents a type that was used for propane prior to the advent of the very large tank cars that came along in the 1960s.
Here is the Atlas 8206 UTLX 11,000 gal tank car.
