I remember people talking bout this ages ago (like, 20 years ago). You keep two bart cards going, and use the same one to enter and leave the same station. If there's a same-station charge, you can use it to enter/leave a nearby station, avoiding the higher fee for longer trips.
Is there a possibility that the same-fare charge is kept around to prevent this hack?
Well, if you have someone going the opposite route, you could swap tickets at either endpoint (or at a transfer station in between.)
> Is there a possibility that the same-fare charge is kept around to prevent this hack?
I'm pretty sure that's why the excursion fare exists.
Obviously, this is not a realistic solution - building new exit gates is simply not worth it.
Practically, such behavior if done repeatedly can be detected as an anomaly in card usage patterns, and a human reviewer can surely figure out what you're doing.
Absent some sort of ranged tag detection (e.g., NFC or RFID), exit determination is exceedingly difficult. I'd argue that NFC/RFID operating beyond a few cm range are themselves a major infosec threat and privacy invasion.