Imagine all the fare inspectors you don't have to pay, all the fare collection equipment and technology, contracts to software companies to make and integrate a mobile app, credit card processing fees...
It raises an interesting question; if it were feasible to implent without massive privacy concerns, would you support income-proportional fee structure for government services? I'm imagining below a certain cutoff income-bracket, everything would be free.
Caltrain and Metrolink (California, USA regional trains) have the right idea. Low income Americans by and large carry EBT cards. They give a 50% discount on tickets when scanned. Of course this could be technologically easily made free but it's a start and the remaining challeges are financial and political.