Once you board the train and find your seat, be prepared to show some paperwork to the conductor when he or she comes through the train car. Failure to have the proper paperwork (or failure to have it properly validated pre-boarding) can result in a fine.
If you're traveling with a train ticket:
You must show ID (typically a passport) and your ticket (with PNRs) to the conductor the train. You can do this on your personal device, such as a smartphone or tablet, or have your ticket printed out. For trains that require seat reservations, you must also show your reservation.
If you're traveling with a rail pass:
You must show ID (typically a passport) and a rail pass that has been validated before boarding the train. For trains that require seat reservations, you must also show your reservation.