This will only show if you have the following requirements:
- iPhone 6 or later
- Apple Watch Series 1 and 2 or later
- The latest version of iOS and watchOS
- An Apple ID signed in to iCloud
- An eligible payment card added to Wallet
- Be in a location that supports transit cards with Apple Pay
Source: Pay for transit with Apple Pay if Express Transit mode isn't supported or enabled - https://support.apple.com/en-us/HT209545
Current locations with Express Transit mode support:
- Beijing: Beijing Transit cards
- Shanghai: Shanghai Transit cards
- All forms of transit that accept Suica
- New York City: Metropolitan Transportation Authority (MTA) accepts payment cards in Wallet at select stations.
- Portland: TriMet accepts transit cards and payment cards in Wallet.
Source: Where you can ride transit with Apple Pay - https://support.apple.com/en-gb/HT20795 8