Insp. John Brewer with the Lower Mainland District RCMP said a large group of young adults was socializing on the waterfront near the north end of Old Yale Road.
He said a handful of them, who had been drinking, tried to jump onto a slow-moving Southern Railway of B.C. train that was switching tracks around 12:30 a.m. PT Saturday.
"One had a mishap and fell under the train, and suffered an injury to her arm," said Brewer. "It looks like now we've had an amputation to that arm. The injured person has been rushed to hosptial."
A friend of the injured woman, who didn't want to give her name, said the victim, who has a young baby, was alert when paramedics rushed her into an ambulance.
The friend said the 19-year-old wasn't immediately aware that she'd lost the arm, saying only that it felt numb.
Brewer said investigators are still trying to figure out exactly what happened.
"We'd certainly like any witnesses to come forward and talk to the Surrey RCMP," he said.
"Obviously we don't want anybody to try to fool around and try to get a train while it's moving or anytime when it's on the tracks. Just leave the trains alone," said Brewer. "It is illegal to interfere with trains."