TechCrunch:

Apple Pay is hitting select stations this Friday, May 31. When that kicks in, riders will be able to swipe their iPhone or Apple Watch to catch a ride.

And:

The kiosks are actually active, at present, but using them requires a software update — iOS 12.3 and watchOS 5.2.1, respectively. Then a debit or credit card needs to be associated with Express Transit in Apple Wallet, using Face or Touch ID. Once installed, it should work on the iPhone 6s and SE or later, along with the Apple Watch Series 1, 2 and 3, using NFC to get you in.

Can’t wait to try this myself. To me, this is the best example of Apple Pay cutting down on payment friction, joining all the other NFC solutions out there, but with the advantage of being tied to your Apple Watch. No need to pull your phone or metro card out of your pocket.