Make sure your Apple Watch is up to date, too.
Open the Watch app on your iPhone, select the General option, and choose Software Update. Your Apple Watch must be running on watchOS 5.2.1.
Then, open your iPhone's Settings menu and choose Wallet & Apple Pay.
If you've ever added a credit card to your iPhone, it's the same menu option.
Choose "Express Transit Card" and select the card you would like to use.
The Express Transit Card feature is different from the regular Apple Wallet because it doesn't require your passcode or Face ID to work.
Then, authenticate with Face ID, Touch ID, or your passcode to confirm that this card will be used for Express Transit.
That's it! You're ready to ride.