Monzo should send a push notification to iOS users who regularly use Apple Pay for public transit. many people do not know you can set up express cards which negate the need for biometric or password authentication at the gate line.
It’s not compatible with all iPhones, only the 3 most recent generations.
Also, I think TfL support for Express Transit isn’t live yet? It’s for other places currently.
It is compatible with iPhone SE, iPhone 6s, or iPhone 6s Plus or later.
You are thinking of reserve mode where express cards work when the battery has died.
You are right about TfL. I did think it was enabled for UK already.