I found this and two others similar the other day in Maine, and understand that they were railroad signs--I assume this one (with reflective circle screwed on top of the painted green surface) signaled "go". Super matter of fact, with heavily nailed and dinged up surface, and nice green on green geometry,  I like it as a readymade painting of sorts, good hung on the wall by itself, and even better with the other two! 

18" x 18" x 1 5/8". Some paint loss and roughness aging top and bottom edges at center. A hook or two or a wire could be added on the back for hanging.