tank car

A tank car is a specialized railroad car built around a large cylindrical or elliptical tank, typically made of steel or aluminum alloy, designed to carry liquids and gases in bulk quantities. The tank itself is the load-bearing structure, welded directly to a steel underframe fitted with couplers, trucks, and brakes. A single tank car can hold between 10,000 and 34,000 gallons depending on its class and the density of its cargo, making it far more efficient than flatcars with drums or boxcars with drums for high-volume transport of uniform commodities.

Tank cars come in several functional types. Covered tanks transport crude oil, fuel oils, and other petroleum products; open-top tanks carry liquids that do not require vapor containment, such as molasses or liquid sulfur; pressurized tanks move compressed gases like propane, ammonia, or chlorine at pressures up to 600 pounds per square inch; and insulated or heated tanks preserve temperature-sensitive cargo like asphalt or molten sulfur during transit. The choice of tank material, thickness, and lining depends entirely on the corrosivity and volatility of the contents.

Regulation and Safety Considerations

Tank cars fall under strict Department of Transportation (DOT) and Association of American Railroads (AAR) classification. A car designated for hazardous materials must meet specific structural standards, including reinforced valves, emergency discharge systems, and tank car specification markings printed on the side. Accident damage, corrosion, or valve leakage can render a tank car unfit for service; many carriers employ eddy current testing and hydrostatic pressure tests on a regular cycle to verify tank integrity.

Loading and unloading occurs through top-mounted hatches or bottom-mounted valves, depending on the design and cargo. Vapor recovery systems are often connected during discharge to capture flammable or toxic fumes. The static electricity risk during loading of flammable liquids is real; improper grounding between the car, the loading rack, and the supply tank can create ignition hazards.

Tank cars are measured by their tare weight (the weight of the empty car itself) and their nominal capacity. A 30,000-gallon general-service tank car might weigh 32 tons empty and can carry up to 90 tons of cargo, making the total weight roughly 120 tons, which approaches the federal limit for a single car on standard rail gauge. This weight distribution has direct impact on locomotive assignment, train speed, and brake system sizing.