CalTrain will let you tap in to the system without even minimum entry fare as long as it's $1.25 (the minimum fare is more than $1.25), and will allow you to tap out of the system even though this will draw your Clipper negative by almost the whole fare. This makes a certain amount of sense for CalTrain, given their affluent market, tech -savvy riders with little patience for Clipper's clunky payment propagation, and the fact that their gateless platforms can't force a person to tap out so they have no reason whatsoever to discourage a tap-out. Their best play is to hope the rider is honest. (Tap-ins are patrolled by fare inspectors who can read the Clipper card).
BART won't let you in the system unless you have the minimum possible fare for a short journey. (Caltrain will). However if you out-range the money on the card, BART used to let you exit. Now, they require cash to be paid at an addfare machine.
An interesting wrinkle in this is that Clipper can be quite slow propagating money contributions to your card. For instance if you use their web site, you are told to wait 24 hours for the value to attach to the card. Even worse, the auto-reload has this same restriction, and it fires at only $10 with no way to raise it. You can easily spend more than that in a single day with a bus+rail round trip, so it's quite possible to get stranded with Clipper.
BART once allowed the card to go negative, but will now require you add cash at an Addfare terminal in the station.
- At this writing, the Addfare machine is not capable of adding balance to your Clipper. Internally, it is seeing that your card is $2.40 short, asking you to pay $2.40 cash, and then telling the faregate to let your Clipper card out even though it is $2.40 short. The Addfare machine cannot refill your Clipper and is administratively limited to $5 change return and BART keeps any change above that, so if don't have small change, you have to potentially let BART keep most of a $20. (Or be reduced to panhandling change or the break of a $20 from passengers).
Prior to that, BART's policy conformed with other Clipper operators to allow one to draw the card negative. There was a problem with people buying free transit cards, loading $2 on them, taking a $8 ride to the airport, and throwing the card in the trash (not least, accidentally by tourists). Hence a purchase price for the card. This is also considered fraud, and a visual indicator showed on a faregate when you exited negative; that was there for transit police. Of course, that played badly with Clipper's slow refill process.
BART dabbled with a regional pass which was a monthly flash pass on all systems except BART, where it had a fare dollar value. It couldn't eat the card on your last trip, since you needed it for a flash pass. So it would give you your last trip for the remaining value on the card, even if it was 5 cents.
Also, on BART's own BART-only farecards, have always kept you prisoner, or to be more precise, made you backtrack to a station you could afford. That philosophy was not compatible with CalTrain, which does not have entry-exit control at stations (nor transfers) so it views a journey as when you enter or exit the train. Caltrain certainly didn't want you undoing an overrun by taking a second train ride for free, so BART's "don't let them out" answer didn't work for them. That is why BART tried the "let you out, go negative" strategy; to harmonize with Caltrain. Thy reversed that recently.
BART is now distancing themselves from their own farecards, and are now herding customers into Clipper.
- Clipper has a bizarre response to a credit card decline. They blacklist the card, which means it will now not work at any faregate until the issue is resolved and that fact propagates out to the terminals.
With today's eyes, you would expect Clipper to sync in real time with Clipper servers using G4/LTE cellular data networks. Actually, at the time Clipper was designed, cellular data networks were not available with the needed ubiquity coughRicochetcough. They did not see a reliable, real-time way to interchange server-side data, so they had Clipper pads store the data locally, and do synchronizations with the servers at night. The readers literally kept cached data on every Clipper card. This did not happen in the small front-end; that was cabled to a larger unit on the bus underside.
Further, this technology had to work with NFC cards available at the time. Early Clipper cards still work, so they at least have backward compatibility.
Even with the data networks today, Clipper does not push every transaction out to every terminal in real-ish time. So if it deducts $5.60 for a BART ride, the Muni bus up on Market street has no source for that data but the card itself. Nor can it communicate in real time with Clipper HQ, for the same reasons you can't get cellular data everywhere. Clipper is not a true stored-value system in the sense that the card is not the sole storer and final arbiter of value. It's mainly kept at the servers, with fairly complex synchronization issues.
The card agencies (often NOT the transit agency itself) are certainly aware of the fraud numbers, since someone is left holding the bag for that, and this turns into agenda items on the Board meetings. Ultimately, they figure the best strategy to counterbalance the opposing forces: fraud vs getting reluctant riders to adopt the system. I have seen agencies across the nation intentionally risk fraud to spur adoption of the cards. Whether that makes sense depends on the numbers.