They email the ticket to you as a PDF so you can print a paper copy as well if you want.
Quite easy to take screenshots of a ticket incase you have no signal, and a charger onboard. Prep is everything.
I'm not sure I trust e-ticketing. I've heard many stories about people who've been unable to show their e-ticket on the train due to a dead phone battery or not being able to connect to the internet who've had to buy a new ticket. Cold.