Authorities say the Colorado State University student was with three other people between the ages of 17 and 25.
Longmont Police say the group was looking to catch a ride to Fort Collins at around 1 p.m. and thought the northbound Burlington Northern Santa Fe train would be the cheapest, easiest way.
When the 17-year-old girl tried hopping on to the train, she slipped and caught her legs between the train's wheels and the tracks.
"It's one of those terrible things that young people who think they're invincible don't think about," Sgt. David Orr with the Longmont Police Department said.
Another teen, a 17-year-old boy from Fort Collins, tried jumping on to the train, but slipped as well. He only suffered minor injuries. Police say he was dragged about 10 feet.
A third person, a 21-year-old from Broomfield, was the first to try the jump and actually made it on the train. Investigators say he left the scene.
The fourth person, 25-year-old Charles Hamilton of Gillette, Wyo., saw what happened to his friends and decided not to take his chances. He reportedly pulled the injured girl from under the train and to a parking area. He was not hurt.
Two of the people involved, including the one who was dragged by the train, were cited for trespassing.
The 17-year-old girl was initially taken to Longmont United Hospital, but was later airlifted to Denver Health Medical Center.
Longmont Police Cmdr. Jeff Satur says he believed the girl was conscious and able to talk.
Satur said he didn't know if the female would be cited.
"She's got enough issues to worry about," he said.
Train speed limits through Longmont are 25 mph. It wasn't known exactly how fast the train was going at the time of the accident.
(KUSA-TV © 2011 Multimedia Holdings Corporation with The Associated Press)