[dragonwriter]

> Isn't there a hack/scam where you could (in the absence of a same-station charge) pay less or nothing to ride bart?

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.