Sometimes its best to just keep quiet, esp if you are on an airport train, and these people doing this, have got nothing to loose, but you have a flight to catch.
Brisbane trains (apart from the city centre ones) have an honesty system, and its just the Translink gocard reader, on a pole, and anyone can free ride, they play it, usually the train has got no staff roaming around.
If the people you call out, cause you trouble, ie, pull a knife, or something, its you who are the looser, as you do not have a self defence weapon, as you would need to clear dom aviation security.
If you cause them trouble, they might pull the emergency stop red button box, and the train stops, thus, you will loose your flight.
Just keep quiet, do not look, do not say anything.
Brisbane (Airport) trains, are not specific for airport services, so the "clientele" are anyone/can be anyone.