I don't know what their purpose is, but one time a CTA or security type person chased a kid (18 ish) onto the train/car I was on at Belmont, yelling at him for not paying the fair and telling him to get off the train. The kid refused, claiming that he paid, and would not move.

The CTA worker removed the muzzle from the dog, and it was pretty terrifying. I mean that dog looked ready to go and destroy the kid, but was waiting for the kill word, which fortunately the worker didn't give. It was a relatively long standoff though that lasted about 5 minutes. I know that doesn't sound like a long time, but it sure felt long.

Everyone else but the kid left the train, and finally the kid left too after those 5 minutes. It seemed like a lot of work and not really worth it to delay the trains, cause everyone to get off, threaten a kid with death by dog, and whatever else just because he didn't pay his fare.