Her response was to haul him down by his belt buckle, "read him the riot act", and ask him whom she should call when he was decapitated by a tunnel.
"My persona changes, if I can put it politely."
The first female guard in the Taieri Gorge Railway's 29-year history does not take prisoners when it comes to the safety of her passengers - even when they are more than a little the worse for wear.
Ms Trevathan went solo last week for the first time, acting as guard to 83 passengers travelling from Dunedin to Middlemarch and back.
It has required nearly two years of study for her to take the step from platform crew - mostly responsible for cleaning - to guard with primary responsibility for the welfare and safety of trainloads of passengers.
If a problem did occur, she was in charge of solving it, she said.
Two of her safety concerns are unaccompanied children on footplates between carriages, and the possibility of a passenger leaning too far out the window.
"You may be conversing with people, but your eyes are everywhere," she said.
She has spent time as an assistant guard, gained two Ontrack qualifications and, as a result, is a qualified shunter.
She finds charters and trips to and from barn dances most "interesting", with passengers locking themselves in toilets and keen to climb on to carriage roofs.
Ms Trevathan's great-grandfather helped build the rail line north of Dunedin and, as a child, she and her mother used passenger services frequently between Dunedin and Waitati.
She began her working life at the Apple and Pear Marketing Board, in Anzac Ave, and spent 10 years as a tour guide at Speight's Brewery until being made redundant.
A widow with two adult children, she was employed at Hillside Engineering before becoming a volunteer, and then employee, at the Taieri Gorge Railway.
At the end of her first trip in charge, with passengers including a Chinese tour party and cyclists returning from the Otago Central Rail Trail, Ms Trevathan said it was "an excellent day out".
"No dramas, no hassles; went very well."
