While it's not the first US use of an iPhone app to substitute for a paper ticket on a rail line -- Amtrak has been doing this for several months -- the Massachusetts Bay Transit Authority (MBTA) has just rolled out the new mTicket app (free) to allow the purchase and display of digital tickets on the South Shore line.
The MBTA notes that about half of its 140 commuter rail stations don't have ticket booths or machines, so riders have to resort to the time-consuming task of purchasing a tick
