Apple Pay already works for pay-as-you-go on the CTA here in Chicago. They added NFC readers to all the 'L' turnstiles and buses for their new "Ventra" payment system earlier this year. I tried it last week and it's pretty cool.
The downside is that you can't use passes with it yet, which sounds like the problem Apple is working with the vendors (Cubic, in Chicago's case) to solve. Right now you would need the device account number. As far as I know Apple keeps this tightly locked down, and even if they did Ventra doesn't have a way of adding regular (i.e. non-Ventra) cards to your account yet for adding passes online. You might be able to load passes on it at a vending machine, but that's not very convenient.
One upside, though, is visitors can use their iPhones to use public transit and avoid Ventra's ridiculous $5 new card fee for a card they'll likely only use for a few days.
The downside is that you can't use passes with it yet, which sounds like the problem Apple is working with the vendors (Cubic, in Chicago's case) to solve. Right now you would need the device account number. As far as I know Apple keeps this tightly locked down, and even if they did Ventra doesn't have a way of adding regular (i.e. non-Ventra) cards to your account yet for adding passes online. You might be able to load passes on it at a vending machine, but that's not very convenient.
One upside, though, is visitors can use their iPhones to use public transit and avoid Ventra's ridiculous $5 new card fee for a card they'll likely only use for a few days.