Answer:
Actually, railroad tank sizes differ as much or more in volumetric capacity than their road-going counterparts.
The nominal sizes range from 15,800 US gallons to 33,600 US gallons. Importantly, in context, American Railcar is not the only manufacturer of railroad tank cars; other manufacturers offer rail tank cars in a variety of additional sizes.
Your question isn't quite as open-ended as "how long is a rope," but it's close.