The metrocard machines are interesting not just because they are easier to use then any other subway kiosk I’ve encountered but that they are unhackable. People have been trying to beat them since they were first installed and the closest anyone ever got, that I am aware of, is if you crease the card in just the right way so that the turnstile can detect the magnetic signal but bit read it entirely, it will let you through after swiping four times. Though supposedly the MTA cops know this trick and look for people with creased cards - in minority neighborhoods anyway. The cards themselves seem to have no stored value but instead point to a record in a central database.