What type of materials do cryogenic tank cars transport?

Cryogenic tank cars are specifically designed to transport cryogenic liquids, which are substances that have been cooled to extremely low temperatures, often below -150 degrees Celsius (-238 degrees Fahrenheit). These liquids include materials such as liquefied natural gas (LNG), liquid oxygen, and liquid nitrogen, which require specialized containment to maintain their low temperatures and prevent vaporization.

The design of these tank cars incorporates insulated tanks to keep the cryogenic liquids in their intended state, preventing them from boiling off and ensuring safe transport. They are fitted with safety features to handle the specific risks associated with transporting substances at such extreme temperatures.

In contrast, dry bulk cargo refers to materials like grains or powders that are transported in large quantities but do not require temperature control, while compressed gases are typically transported in atmospheric conditions rather than as cryogenic liquids. General freight encompasses a wide range of goods that do not fit into the specialized categories of cryogenic or bulk transportation. Thus, the primary function of cryogenic tank cars is to transport cryogenic liquids, making this the correct answer.