What does ‘They’re on’ mean in the following context?
Kristin: So, I got on the train. I go in the back like I usually do because I have a pass. Sit down, we go one stop. We’re actually coming up to 18th, the next stop. I see the ticket checkers, for lack of a better word.
Joe: Yeah, I know who you’re talking about
Kristian: People who are always checking for passes