Think about others, the train driver, the passengers. They don't want that trauma.
As for the reliability, speed would be the biggest determinant of reliability and cleanness, because KE = MV^2, so you double the speed, you quadrupal the energy. You really want something going at least 50mph for a quick kill. Once you get below 40mph there's an outside chance you get horrifically injured but survive.
The typical speed of a freight train in the US is between 30-60mph. Up to 70mph in some areas but you'd have to research that. Plus the driver might brake when he sees you so that's going to slow it down a little.
In Europe you have much better options.
But overall i think it's a terrible thing to do to the train driver. Also survival instincts may cause you to bodge it and only get a glancing blow.