One man was seriously injured after four people jumped from a moving GO train early Monday.

The incident happened at about 12:30 a.m. as the eastbound train approached Danforth station on the Lakeshore East line.

Metrolinx spokesperson Anne Marie Aikins said someone pried one of the train's doors open, which triggered a safety device that prevents the train from accelerating. The first person jumped from the moving train and three others went outside to help him.

Aikins said it's not known how fast the train was travelling but said it was moving "fast enough that one of the people received serious injury."

Paramedics loaded the injured man onto a stretcher and put him back onto the train. He was taken off the train at Danforth station and transported to hospital by ambulance. He is expected to survive.

Based on eyewitness accounts, Metrolinx initially said the four men jumped around the same time from the train. Hours later, Metrolinx said it appears the first person jumped out before the other three left the train to help him.

Aikins says the investigation continues and will determine whether charges are warranted.

"I don't know the why, and I'm not 100 per cent sure about the how — we're starting to piece that together, how he did it," she said.