~30km/h is enough to crush your head and usual passenger trains can go up to 200km/h. Also depends how fast they are in your area
if you want to die, you must be hit in the skull or neck for maximum reliability. Don't lay on the tracks to get your legs cut off, but in a position that exposes your head for the train.
I've seen videos of people simply standing still on the tracks and waiting for train to come. This also works, but if it's higher speed train you must know that parts of your body will be everywhere.
Also hearing the train from the distance can be stressful, knowing you will die soon. So to fix this issue you can wear headphones with loud music (however if you're laying on the tracks you'll be able to feel vibrations anyway).