Well it depends. I used to be a conductor for NS so I've been through some accidents, been trained in safety, etc.
There are a lot of people who choose suicide by train. Most of the time you do not die on impact with the train, basically you have to be cut by the wheel. Most people end up under the train, get their clothing snagged and are dragged. They end up very alive, but with deep gashes from the equipment under the train and debris.
Now, one of my friends actually fell asleep laying perpendicular to the tracks waiting for a train to come. We don't know exactly what happened, but somehow the train pushed him to where he was parallel to the tracks and dragged him along. Of course he had part of his head sliced open, and other very serious injuries. He had multiple brain surgeries, and was in physical therapy for over a year to learn how to walk again.
You can still die from being dragged under the train though. I had a friend die up in Montana, he slipped off the back of the train and was dragged to death. The trains up there go super fast so I'm sure the speed and how long it takes for the train to stop has an impact on the fatality.
Then there is the threat of dying from just having a limb cut off. One of my friends was getting off of a train, lost his footing and got his leg cut off at the knee. He died from lack of blood before anyone found him.
Then there is the threat of being "coupled". Sometimes in the train yard, when they are shoving cars back and forth the cars are very silent. If you happen to get caught in the machinery of the coupling you usually don't die immediately. There have been many cases where the person died when he was uncoupled.
So to answer your question, it's all a matter of physics, luck, and how the impact occurred.
