What does 'ther

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

Joe: right

Kristian: THEY’RE ON and I thought…

WHAT DOES THAT MEAN?

they’re = they are

http://en.wiktionary.org/wiki/they’re

Well, I know that, I just wanna know what ‘THEY ARE ON’ means in this context, I didn’t understand.
Thanks anyway

From your context I guess she means "they (the ticket inspectors) are on the train, in the carriage.