Why do they call the railroad yard security guard the "bull?"?

Question

I've ridden the rails before and I was chased by a railroad yard bull once, but I wonder where he got his name.

Answer 1

old school

cops always called bullls

Answer 2

not really sure where, it goes back to the 1800's.

I got this snippet off a website but still no history of the term itself, sorry.

BULL—Railroad policeman. Also called flatfoot or gumshoe, but the distinctive railroad terms are cinder d**k and 'bo chaser