JBL Hazardous Materials Practice Test

Understanding how pressure affects tank car design helps here. A non-pressurized rail tank car is built to carry liquids without containing them under any significant pressure. The cargo’s vapor pressure is vented to the atmosphere, so the internal gauge pressure stays near zero and well under 25 psi. This is what defines it as non-pressurized. In contrast, a pressurized car is designed to keep cargo at higher pressures and includes relief devices to manage that pressure. A vacuum car operates by pulling the contents below atmospheric pressure to reduce heat transfer, so its internal pressure is negative relative to the atmosphere rather than simply low positive pressure. Cryogenic cars are focused on very low temperatures for ultra-cold liquids, with pressure behavior tied to boil-off and insulation rather than a basic low-pressure design. So, the type that typically operates at less than 25 psi is the non-pressurized tank car.