I believe that's a stationary horn, "aimed" down the roadway toward approaching traffic. Above it would be a signal indicating to an approaching train that it's working. Since the horn is "aimed," it projects the sound through a narrow arc, thus being less annoying to those around who aren't directly affected. With this in use, the engineer doesn't blow the train's horn. The only place I've experienced it is on the BNSF at Escalon, CA; it's pretty weird to watch a train approaching and have a horn start blowing right behind you.