I used to work for the Frisco railroad (now BNSF) back in the late 70's and early 80's. Even back then all automobiles were shipped in enclosed flat cars. There were two types, bi-levels for taller vehicles and tri-levels for automobiles. The outer shell of the rail car is made up of sheet metal perforated every few inches with 1-2" holes. It keeps large rocks and similar items from striking the autos. You would be amazed at how many kids throw things at the train as you pass by. The perforation allows air flow in and around the autos as well as train crews to see if it is loaded or empty. This is where the brake dust comes from. There is quite a variety of rolling stock at different levels of disrepair. All it takes is one rail car to have a sticky brake shoe and the whole train is bathed in dust and smoke. It was very common to have a brake shoe stick and the crew would have to stop the train and set the bad car out on some siding for a "carman" to come and repair it. Next train by would pick it up.
Next time you see a train go by take a look, you will see these tall, long flat cars with galvanized sheet metal sides with folding doors on each end. That is an auto transport.
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