Presumably it will be similar to an airline e-ticket or boarding card. You can usually download to a phone or print out on paper, then present the barcode or QR code.
I was on a train in Germany once, when the conductor came along and asked for tickets. A couple near me said they had the tickets on a mobile phone but the battery was flat. I don't know the outcome but I know the conductor said 'it's your problem'. So it would always print even if I had downloaded to a phone.