New York's Metropolitan Transportation Authority initially promised that smartphone-friendly train ticketing would roll out by the end of 2016, but (unlike some trains) it's ahead of schedule. Governor Cuomo has revealed that the MTA eTix app will be available to all Long Island Rail Road and Metro-North Railroad customers by the end of the summer. The rollout will happen in phases. You can already buy tickets for LIRR's Port Washington Branch and the Metro-North Hudson line. By August 22nd, there should be a "full system wide implementation" on both railways -- you usually won't have to worry about carrying physical tickets as long as you have an Android phone or iPhone.
The approach won't be completely seamless. You'll have to wait until next year to use a single app and account for trips that involve switching to and from New York City's buses and subways. However, eTix could still be a huge convenience if you like to travel across southern New York state... or just hate carrying easily-lost pieces of paper.