What type of structure is a rail tank car mounted on?

A rail tank car is mounted on axles and wheels, which are essential components for its movement along railway tracks. The axles provide the necessary support for the tank car's weight and stability while enabling it to roll smoothly. Wheels are designed to fit securely on the rails, allowing for safe and efficient transport of liquids—such as chemicals, petrochemicals, or other bulk liquids. The combination of axles and wheels creates a robust structure that can withstand the rigors of rail transport and ensures that the tank car remains secure during transit.

The other options do not accurately reflect the structure of a rail tank car. For instance, a wooden frame would not provide the structural integrity needed for heavy loads, and a concrete base is impractical for mobile transport. A plastic chassis would not offer the strength and durability required for the rigors of rail travel. Therefore, the correct choice highlights the essential components that allow a rail tank car to function effectively within the rail system.