[Fej]

NJ Transit uses QR codes for all train tickets and it works well enough. It's not quite as seamless as a tap card but NJT has moved largely to mobile ticketing and they're in the odd position of having to scan tickets both at turnstiles and by train crew (handheld scanners); cards would be awkward in the latter case. The paper tickets used to have magstripes but once they adopted QR codes for the mobile app, the tickets lost the magstipe and gained a printed QR code.

So the central server model is practical. The user's device has to have an internet connection at some point to activate the ticket within a reasonable period before using it but the connection doesn't have to persist after that. I don't know how their handheld scanners work in the Hudson River tunnels where there is no cell service but they do, so long as the user activates their ticket before the train departs.

[user_7832]

> cards would be awkward in the latter case

Do you mean it would be hard for staff to scan cards? Here in the Netherlands both qr and cards (travel cards + bank cards) can be used at stations, and are read by the staff on the train.