Railroad Signs

[green_elite_cab]

everyone is always trying to get the railroad sginals and other dealies, but i notice on some tracks there are signs with random letters on them. I can't remember any particular one except for a W or something with other letter with it.

What are the signs for, and where would i place them?

 

[N Gauger]

I know "W" is for Whistle.. it's where the whistle signal should be blown or started being blown...