As I tried to do some research into the history of railroad signals in order to date--and to fully understand!--this sign, I've realized it is a vast and complicated history I don't have enough knowledge, or patience, to dive deep into!  I did learn that the use of train whistles seems to have originated in England in 1832, when a stationmaster suggested that the trains should have an audible signaling device, and thus a local musical instrument builder was commissioned to create a steam-powered whistle, then known as a "steam trumpet". In North America, a wide variety of different types of whistles were developed, and signal codes along with them. The code described here seems generally in keeping with what else I've seen, I believe with the conflicting meanings of 1 and 4 whistles relating to whether issued as short or long sounds, as with morse code. 

Anyway! I really love the graphic organization of the information on this sign, and the language itself: Stop, Back, Check, Strong, All Right, and, best of all, the final instruction that 2 whistles or bells shall always mean BACK, irrespective of prior signals. A perfect sign to hang near the front door of a busy household!

Late 19th century I'd guess. I have not found another like it out there. Painted using stencils I believe, white on black. Scratches, some paint loss, and rusting around the edges as pictured, but not fragile and still sharp looking. Holes in the corners for hanging and a fifth hole at lower center. 14 1/4" t x 10 1/8"w.