Which of the following is NOT a characteristic of pressure tank cars?

Pressure tank cars are specialized rail cars designed to transport liquids, gases, or both at high pressures. These cars have several essential characteristics that ensure safe and efficient transportation.

Bottom unloading piping is not typically characteristic of pressure tank cars, as they are designed primarily for pressurized transport and unloading often occurs directly from the top or via side openings. In contrast, many other tank cars do employ bottom unloading systems for gravity drainage but pressure tank cars prioritize maintaining the integrity of the pressurized environment.

Insulated walls are a common feature in pressure tank cars that transport temperature-sensitive materials, ensuring that the contents do not change state due to temperature fluctuations during transit. This insulation helps maintain the material's properties.

Pressure relief valves are integral components of pressure tank cars. They are crucial for maintaining safety by preventing excessive pressure build-up that could lead to catastrophic failure.

Safety systems for leakage, including various monitoring and containment features, are implemented to mitigate the risk of spills or leaks. These systems contribute to overall safety in the transport of hazardous materials.

Thus, recognizing that bottom unloading piping is generally not a defining characteristic of pressure tank cars helps clarify why it stands apart from the other listed features, which are integral to their design and functionality.