Your basic tanker car holds 22,800 gallons, though there are many different designs. But you can't fill it full. You need to give it some head space, called "outage", and that varies on what you're shipping. It's 5% for hazardous materials and 1% for non-hazardous ones by default, and specific materials have other standards. You need to give it some room to expand with temperature, or the whole thing may explode. Sources: https://www.procor.com/helps/how-read.