What is this?

[glatt]

Originally Posted by Lamplighter

Trucks in mountainous areas carry "emergency drop chains" for icy roads and runaway trailers.

Are they some sort of "emergency drop skids" for railroad freight cars ?

You're really close. Close enough that I'd say you have it.

They are called skid shoes, and heavy wagons going down hills used them to slow the wagon and also keep the brakes from wearing out and the metal tires on the wheels getting a flat spot worn into them as the wagon skids down a hill.

These would hang by chains on the side of the wagons, and you would stick them under the wheels at the top of a hill, and just skid down the hill on them.