Amtrak, the US Rail system has some of the most archaic ticketing systems of any rail system in the world. One element of their system is that you have needed a paper ticket to ride.
As of last Monday, Amtrak passengers may print out their own tickets at home or download them to their smartphone or tablet. Yes, E tickets have arrived on trains, only years after airlines abolished paper tickets.
The system was trialled in California and had a massive uptake. I have been generally using the electronic ticketing machines at stations which has always worked well. They are quite fast but this speeds things up a little more. The lines at some Amtrak stations (eg New York Penn station). to speak to a human are ridiculous