Somewhere around here, I think I still have a souvenir BART ticket with a nickel left on it, from a school outing back when BART was new.
Back in the day, you'd run the ticket through a money-taking machine, which would record the balance on the magnetic stripe; the turnstile leading to the train would record your point of origin, and the turnstile at the exit would calculate the fare and update the balance on your ticket.
I suppose there must have been some time-stamping going on, too... right?... to prevent people entering and exiting the same station at no charge, several hours apart, using one ticket to do that at the home end and another at the office end.
That system allowed the use of standalone turnstiles; the exit turnstile, after reading where (and perhaps when) you came in and how much money was left on the ticket, could simply look up the fare and update your balance.
In these days of ubiquitous networking, though, are such standalone turnstiles really necessary?
Apparently either they still work that way, or subway fare-system architects have a serious blind spot.
Y'see, with the balance being stored on the mag stripe, BART tickets were fairly easily hackable, and it didn't take people long to figure out how to write larger balances to them without the bother of actually putting money into a fare machine.
And now: someone's gone and found a way to reset the balance on a state-of-the-art contactless fare card.
Wha...?
They're still storing the balance on the card?
The turnstiles aren't networked to a server that keeps track of the balances, leaving the card with nothing to carry around but a unique ID?
How does this make sense?
(Yeah, sure: safety reasons! We need to make sure people can leave even if the network is down! Er, or maybe if the network is down you just let people leave and lose the fares? Or record their card IDs for later processing, on the assumption that most of them are good for the fare?)
Afterthought: Inspiration strikes! It can't be done the smart way because Privacy. Imagine the potential for misuse if the subway's Central Computer knew that pass #41187936079 entered the system at 37th Street at 07:54:12.336 and exited at the Oak Avenue station at 08:38:46.047! (I'm assuming that passes are can be bought at vending machines that accept cash, so there's the option of having a pass that's in no way associated with an actual person.)
Given how wound up some people get about cookies, or having web site owners see the search strings whereby visitors arrive, this may be the true explanation. Mind you, those same people happily ignore real privacy violations by the real Big Brother, just so long as they're allowed to squawk about utterly inconsequential matters.
[Yes, the last four digits of the pass number are intentional.]
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