The phrase "railroad cars" refers to the vehicles or carriages that are used on a railroad or train. They are used for transporting goods or passengers from one place to another on the railroad tracks. Full definition
This restaurant is actually an old railroad car, and we were able to request to sit in a private dining car for our dinner. (have-clothes-will-travel.com)
Good for the miners, loading the night's coal onto railroad cars; good for the children, who walked two miles each way to school. (bookbrowse.com)
I didn't know what that was; I thought engineers ran railroad cars. (sciencemag.org)