If no one knows, a close estimate can be made by measuring the car. Subtract a little for the shell thickness, ignore the dished ends, and just use the tube portion, and you'll be fairly close. Then find the nearest capacity on your decal sheet.
I don't know if these are insulated or not. If not, I'd take about 2 inches off the outside diameter. Insulated? Maybe 6 to 8 inches? How thick is tank car insulation?