Steel beam colors

[MountainMan]

Flatbed trailer loads are tied down as a group on top of wooden beams, so I would imagine the same sort of process would be used by the railroads.

This might help:

The difference between flat bed trailer loads and loads on rail cars is that the trailer is loaded, the straps are fastened by the driver and he then drives the truck to the delivery point where he removes his tie downs and stows them back in his truck or the trailer. Since the flat car is loaded at the mill, sent across country on a train that may be interchanged between a number of railroads, and all tie downs, blocking, etc is considered dunnage to be disposed of when the car is unloaded, they would definitely use steel bands rather than expensive nylon tie down straps.

I posted the image to show how the concept works, not to specify the materials used.