A railroad police officer. Often just referred to as a “Bull” by tramps, hobos, and graffiti artists. Their job is to secure rail yards and prevent theft from or damage to railroad property.
When riding the rails, you have to look out for the local Yard Bull.
Yard Bull: define #2
A police officer who patrols train yards looking for train-hoppers, oogles, crusties, etc.
Man, did you see that fat yardbull chase us through the train yard?!