5.2 Rail Tank Cars and Intermodal Tanks

Rail Tank Cars and Intermodal Tanks: Identification and Hazards

Rail transport carries massive volumes of hazardous materials through communities. Because of the enormous quantities involved—often exceeding 30,000 gallons (113,500 L) per tank car—any incident presents severe potential hazards. First responders must distinguish between pressure and non-pressure rail cars and recognize intermodal containers that transfer between rail, highway, and marine transport.

Rail Car Markings and Reporting Systems

Every rail car carries markings that provide responders with key identification clues. Reporting marks are stenciled on both sides and both ends of the car. They consist of letters indicating the owner (e.g., GATX) followed by a car number. If the mark ends in "X", the car is privately owned rather than owned by a railroad company. Capacity markings (in gallons and liters) and tank specifications (like DOT 111A100W1) are stenciled on the sides, indicating design characteristics and pressure ratings. Responders can use reporting marks to obtain the train consist (manifest) from the railroad company or CHEMTREC.

Non-Pressure Rail Tank Cars (General Service Cars)

Also known as low-pressure or general service tank cars, these vessels transport liquids such as crude oil, ethanol, mild corrosives, food products, and clay slurries.

• Pressure Range: Operating pressures are typically under 100 psi (689 kPa), usually ranging from 35 to 100 psi.
• Silhouettes and Fittings: The defining feature of a non-pressure tank car is that its valves and fittings are visible on top, mounted on a small manway cover without a heavy protective dome housing.
• Bottom Outlets: Many non-pressure cars have bottom discharge outlets, a feature not permitted on modern pressure rail cars.
• Responder Safety Traps: A common mistake is assuming that "non-pressure" means zero hazard. A breach in a non-pressure tank car carrying crude oil or ethanol can feed a massive, prolonged fire.

Pressure Rail Tank Cars

Pressure tank cars are designed to transport flammable gases, non-flammable compressed gases, liquefied compressed gases, and highly toxic materials like chlorine, anhydrous ammonia, and ethylene oxide.

• Pressure Range: Typically designed to withstand pressures ranging from 100 to 600 psi (689 to 4,137 kPa).
• Silhouettes and Fittings: All valves, piping, and pressure-relief devices are enclosed within a single, heavy protective steel dome housing located on top of the tank. This dome protects the fittings from being sheared off during a derailment.
• Tank Construction: The tank has a circular cross-section, is built of thick carbon steel or alloy steel, and features rounded, hemispherical ends. They do not have bottom discharge valves; all loading and unloading occur through the top fittings inside the dome.
• Thermal Insulation: Most pressure tank cars are insulated and jacketed. A thermal jacket protects the inner pressure vessel from direct flame impingement, reducing the likelihood of a Boiling Liquid Expanding Vapor Explosion (BLEVE).

Boxcars, Hopper Cars, and Gondolas

Non-tank rail cars also carry hazardous materials:

• Boxcars: Enclosed rectangular cars with sliding doors on the sides. They transport individual packages, drums, cylinders, or intermediate bulk containers (IBCs) of hazardous materials. The hazard is hidden inside, making shipping papers and placards critical.
• Hopper Cars: Dry bulk containers with sloping bottom doors (or gravity-feed outlets). They transport dry, granular materials like ammonium nitrate fertilizer, plastic pellets, or cement.
• Gondolas: Open-top cars used for bulk solid materials, including contaminated soil or hazardous industrial waste.

Intermodal (IM) Tank Containers

Intermodal containers, or ISO tanks, transfer between ship, rail, and highway chassis without unloading the cargo, consisting of a single tank mounted inside a standardized 20-foot metal frame.

Non-Pressure Intermodal Containers (IM-101 and IM-102)

These containers are the most common type of intermodal tanks used for liquids:

• IM-101 (Type 1): Operating pressure ranges from 25.4 to 100 psi (175 to 689 kPa). They transport hazardous liquids, flammables, toxic materials, and mild corrosives.
• IM-102 (Type 2): Operating pressure ranges from 14.5 to 25.4 psi (100 to 175 kPa). They typically transport non-hazardous liquids, food-grade materials, and low-hazard chemicals.

Pressure Intermodal Containers (Specification 51)

Also known as Spec 51 or Type 5 containers, these tanks are built to transport high-pressure liquefied gases:

• Operating Pressure: Typically ranges from 100 to 500 psi (689 to 3,447 kPa).
• Silhouette: They feature a heavy cylindrical tank with rounded ends, and all valves are protected by recessed covers or protective boxes. They carry LPG, anhydrous ammonia, and high-hazard chemicals.

Cryogenic Intermodal Containers

These are specialized double-walled, vacuum-insulated containers designed to transport cryogenic liquids:

• Operating Pressure: Typically less than 75 psi (517 kPa).
• Silhouette: They feature a box-like control cabinet at one end of the frame, housing the valves and pressure gauges. Cold vapor venting is a normal operating function.

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Rail and Intermodal Container Comparison Table