Funny, that sign is just like the ones I see with an image of a deer leaping across the roadway. Or "Warning, falling boulders"... Or the image of a car skidding on ice. Or "high water" when there isn't any.

Why are these signs there? Because what it depicts CAN HAPPEN! Boulders have fallen at that spot before, so it's best to put a sign up for some kind of warning than none at all. Trains do pass through Oceanside Station, and there is the chance that a passenger train could blow through without stopping. It's also a nice reminder for people to keep in mind for stations like Carlsbad or Encinitas...

Dead-head moves are another example, I remember of a half-dozen deadhead moves that occurred when I lived in Irvine that had double or even triple Surfliner sets coupled together blowing through Irvine Station.

Makes me wonder about the sign at the zoo that warns people not to stand behind a Rhinoceros in the case it decides to excrement... I haven't seen it happen so the zoo must be lying!