An oil tank rail car is a type of railroad car or rolling stock designed to transport liquid
(oil/condensate) and gaseous commodities via a railroad.
The evolution of the tank rail car is fascinating and the below highlights the major events of the evolution of the modern tank car:
1865: Flatcars with banded wooden tanks mounted on top are employed for the first time to transport crude oil from the fields of Pennsylvania during the Pennsylvanian oil rush.
1869: Wrought iron tanks, with an approximate capacity of 3,500 US gal (13 m3) per car, replace wooden tanks.
1888: Tank-car manufacturers sell units directly to the oil companies, with capacities ranging from 6,000–10,000 US gal (23–38 m3).
1903: Tank-car companies develop construction safety standards. More than 10,000 tank cars are in operation.
1915: A classification system is developed by the tank-car industry to ensure the correct match of car type to product being shipped. Some 50,000 tank cars are in use.
1920: Welding replaces riveting in tank-car construction, enhancing the safety of cars.
1940s: Virtually every tank car is engaged in oil transport in support of the war effort.
1950: Pipelines and tank trucks begin to compete for liquid transport business.
1963: The Union Tank Car Company (UTLX) introduces the "Whale Belly" tank car.
Any variants exist due to the wide variety of liquids and gases transported. Tank cars can be pressurized or non-pressurized, insulated or non-insulated, and designed for single or multiple commodities. Non-pressurized cars have various fittings on the top and may have fittings on the bottom. Some of the top fittings are covered by a protective housing. Pressurized cars have a pressure plate, with all fittings, and a cylindrical protective housing at the top. Loading and unloading are done through the protective housing.
Tank cars are specialized pieces of equipment. As an example, the interior of the car may be lined with a material, such as glass, or other specialized coatings to isolate the tank contents from the tank shell. Care is taken to ensure that tank contents are compatible with tank construction.
As a result of this specialization, tank cars have generally been "one-way" cars. Other cars, like boxcars, can easily be reloaded with other goods for the return trip. Combinations of the two types were attempted, such as boxcars with fluid tanks slung beneath the floors. While the car could certainly carry a load in both directions, the limited tank size made this unsuccessful.
A large percentage of tank cars are owned by companies serviced by railroads instead of the railroads themselves. This can be verified by examining the reporting marks on the cars. These marks invariably end in X, meaning that the owner is not a common carrier.
Within the rail industry, tank cars are grouped by their type and not by the cargo carried. Food-service tank cars may be lined with stainless steel, glass, or plastic. Tank cars carrying dangerous goods are generally made of different types of steel, depending on the intended cargo and operating pressure. They may also be lined with rubber or coated with specialized coatings for tank protection or product purity purpose. The tank heads are also stronger to prevent ruptures during accidents. The whale-belly type is giving way to higher-capacity, yet standard-width cars.
All tank cars undergo periodic inspection for damage and corrosion. Pressure relief valves are inspected at every loading. Pressurized cars are pressure-tested regularly to insure the integrity of the tank.
All tank cars operating throughout North America today feature "double shelf" AAR type "F" couplers that prevent disengaging in event of an accident or derailment. This all but eliminates the chance of couplers puncturing adjacent tank cars.
Insulated cars (which may also incorporate heating or refrigeration systems) are used when the contents must be kept at a certain temperature. For example, the Linde tank car depicted below carries liquefied argon. Cars designed for multiple commodities are constructed of two or more tanks (compartments). Each compartment must have separate fittings. The lower capacity and added complexity of multicompartment cars means that they make up a small percentage of the tank car inventory.
Outside of North America, tank cars are also known as tank wagons or tanker wagons.