One train does what 120 to 180 trucks might do. Revenue cars are those which earn a railroad money. They include freight and passenger cars. By means of them, the railroad does the job for which it is paid. There are two ways this works.
Several railroads have excess cars which earn revenue through leasing to other roads.
One extreme example is the Iowa Terminal Railroad. Ironically, the Iowa Terminal does not have enough track to hold all its cars at once. Car types are standardized for the types of freight which can be hauled. That is a boxcar, used to handle dry freight and packaged goods. Boxcars can be very colorful, for the owners would paint them as a moving advertisement for their services.