I have no idea what they're called or what they're used for...

[Anonymous]

They are called telltales and they are a warning device for anyone standing on top of a moving train that they must immediately duck. You do not see them today because there are no roofwalks and there is no reason today for anyone to walk a moving train.

[tigerstripe]

It is called a Tale tell or telltale I'm not sure which but they were used during the time when brakemen used to ride on top of cars to set retainers and brakes in preparation for a downgrade run. So a man would be hit by the ropes that would warn him to lay down a tunnel or some other low clearance was approaching.

[Anonymous]

Okay, I've only seen them in use on steam era model railroads. Apparently they were a real feature on prototypes, and I have never seen them in modern times. I've only seen them on model railroads, never in pictures of the prototype, but I'm sure they must've been a real thing, otherwise the modeler wouldn't have bothered putting it on his layout.

It looks like a tall utility pole, with a 90 degree arm mounted on top extending out over the track, with lots of chains or ropes or wires, hanging down from the arm.

My guess is, it's some sort of clearance measuring device. Like "if you're taller than these lines, you're too tall for an upcoming obstacle" kinda thing. I wish I could find some pictures of the real thing and I'd post them.