Staff Sgt. Blair McColl says a group of young people were walking along train tracks at around 12:30 a.m. Saturday.
The train slowed down to a crawl as it approached a switch point before stopping and slowly backing up to change tracks.
McColl says the 19-year-old woman tried to jump onto the train, but she slipped and fell and it ran over her arm.
He says her arm had to be amputated below the elbow, but she's now recovering in hospital with non-life threatening injuries.
McColl says the group had left a party and were trying to keep pace with the train, with some trying to jump onto it.
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