Rail travel is not going to be conditional on having a working smartphone in your pocket!
If you buy through trainsplit the PDF will be emailed to you the email will already come to your phone. Absolutely nothing wrong with printing the PDF off for reassurance but you don't need to do that, you can simply show the PDF on your phone. I used to always print booking confirmations for hotels and travel tickets as well as having it on my phone but stopped printing them around a year ago as I found I never used them and got into the habit of just showing everything on my phone. It does take a but of getting used to and feels like a bit of a leap of faith but seriously I wouldn't look back now.
I've been using QR code tickets from Trainsplit for a couple of years. There's no problem with them for me at least as I have plenty enough IT resources at home. Here's the last one that I bought, (which together with its return part I wasn't able to use, - see date).
At £4.60 each way a refund wasn't worth the bother.
Interestingly, the actual ticket graphic measures 105mm x 79mm so when folded would be 52.5mm x 79mm, i.e. mariginally smaller than the ISO standard for a credit card. This means that if tickets were printed by TVMs on standard 80mm width thermal paper, with a tear off dotted line seperating the ticket section from the itinerary and receipt, passengers could continue to carry them in a card wallet. If the ticket was kept in a windowed pocket, the ticket could be scanned at the gate
without removing it from the wallet.
Using a phone with the same ticket would involve finding the ticket to display, assuming the phone was switched on and not busy on some other application at the time.
So, if the railway was really interested in moving away from the current magstripe card design, it should replce them with something that can be purchased, carried and used by passengers without possessing any IT equipment (including a phone) at all. Then it would be fair to all and get general acceptance. Those who wished to keep their ticketing entirely within the electronic domain would be able to use a smartphone as some here are advocating, and the halfway house that folks like me want with self printed tickets would also fit into such a ticket roll model.
p.s. I assume that those above calling for barcodes on tickets are effectively referring to QR codes which can store more than just a simple number.