Apple's plans to expand support for mass transit now appear likely to hinge around its next big iOS 12 update. Tap Down Under has found code within iOS 12.3 that hints at support for EMV payment cards (think credit cards like AmEx, Mastercard and Visa) within Apple Pay Express Transit, letting you pay for public transportation fares in New York and other areas where EMV is an option. You could set a preferred card for transportation if you don't want to use your usual card for subway rides.
It's not certain just when iOS 12.3 will arrive, although there have already been four betas as of this writing. A finished release may be a few weeks away, if not sooner.
Whenever it shows up, it would be part of a larger plan. Reported leaks claim that iOS 13 will support a much wider variety of mass transit cards, including different takes on cards that require loading value. It might just be a matter of time before your iPhone supports a local tap-to-ride service, whatever the format.