Basically, it works like this: if a shoe company in California ships a boxcar of shoes to NY, a boxcar, say belonging to the Santa Fe carries the shoes to the end of the Santa Fe, gets switched out and picked up by a connecting railroad, say Pennsylvania, and put in one of their trains heading East. When it gets to the end of their line, it may be turned over to another railroad, say the NYC, who serves the customer. All the way, that boxcar is tracked by the number on the sides and ends, for instance,”SF 20345”. Once the shoes are unloaded, the reverse happens, and the car is returned to the Santa Fe on a train headed West. That’s why trains are made up in huge yards, and sorted to head in the direction of the ultimate destination.
Gosh, I hope a professional railroader will jump in and clarify!
