They're on - Tradução em português

Hey folks, I don't understand what ''they're on" means 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

Joe: right

Kristian: THEY'RE ON and I thought (...)