Flanger Signs

Does anyone know what the yellow signs that look like upside down hockey sticks are? You see them frequently along the commuter rail lines. Thanks for any info.

Flanger signs tell snow plowing equipment to raise the flanger when approaching an obstruction in the gauge, most often a crossing or a bridge with guardrails.

They're not always yellow. I think that on the New York Central and some other roads they were painted black on the theory that the only time they need to be seen is against newly fallen snow.