Rail Signs

Iif you're talking about the North American environment then I think it's a whistle sign - ie it's a direction to the engineer to blow the whistle. However if you're talking about Germany it means "Wait".
 
a small white sign with a slash through it meens there is a driveway or crossing coming up so the flanger slearing snow wont tear it up
 
Different railroads have different systems. Your best bet would be to acquire rule books for various railroads you are interested in learning about. (Spending money for information?! Unheard of!)

~BS