EIGHTEEN passengers smelt it on an XPT train and no one can say what dealt it.
But the odour put the passengers in the emergency departments of Grafton and Kempsey hospitals.
The son of a passenger who did not want to be named said whatever his father smelt on Wednesday night gave him double vision, nausea, poor balance and appetite loss.
"Dad got off the train at Grafton," he said.
"He had a bit of balance trouble and was unstable on his feet.
"The next morning he got worse and I took him to the hospital.
"Everyone had to put on masks when he was in the emergency department.
"Thursday morning I saw the paramedics at the South Grafton train station checking two people out."
A NSW TrainLink spokeswoman said 15 people were taken to Kempsey Hospital because of the smell.
"About 10.40pm Wednesday, NSW Fire and Rescue were called to investigate an unusual odour reported just before it arrived at Kempsey," she said.
"Investigations are under way to determine the cause of the odour.
"Once NSW Fire and Rescue provided clearance for the train to operate and relief staff arrived on site, the service left Kempsey at 5am to Grafton.
"NSW TrainLink apologises to customers who experienced delays of up to seven hours into Brisbane.
"NSW TrainLink is contacting all customers to apologise and offer a full refund."
A Northern NSW Health District spokeswoman said three people were put in Grafton Base Emergency because of the smell.
"There was a smell in the cabin," she said. "As a result, they got sick."