Maybe each car was carrying a

Maybe each car was carrying a specific part of a structure,  It looks like wood. Not sure.

If your modeling this specific train, you'd want the "signs".  I've never seen those before, prototype or model,that why I'm thinking it's a train specific thing.

A bigger picture would help, when I use the ctrl and the + sign to enlarge, it gets way out of focus.

Mike.

Car/scrap yard used to do something like this

One of the scrap yards I used go to would do something like that,  Silver sign with a "A" for Aluminum and Orange sign with "S" for other metals.  But they were on the side of the car held in place with speaker magnets and removed before the cars left, I would assume.

I will guess on why this is done, so not saying I'm correct.  The yard has one track that runs down the middle with wrecked autos/cars on each side.  The railroad would pull the full rail cars out and shove empties in.  Rail cars as far as I could tell were never in any order.  Empties were closer to the gate, fulls would be at the far end.  I think it was just the easy way for the railroad to put cars in the scrap yard.

The cars were loaded with bob cats and front end loaders. in most cases I do not think the operators could see over the top of the rail cars and what was in them.  They looked at the sign and used it to dump scrap into the correct cars.  As the rail cars we never in the same place from day to day as more empties would be closer to the gate and the fuller cars would be pushed further into the scrap yard.

There is no bigger

It's from a book and the focus is what it is. Sorry.

Also, the cars were apparently on a moving train, on the NKP main.