Apple Pay for public transit isn't totally new, but it is new to the US. Subways and rails in London, Moscow, Tokyo and Guangzhou all support the ability to pay for fares with Apple Pay. Transit systems in Shanghai and Bejing allow users to add their pre-loaded transit cards to Apple Pay with a service called Express Transit. It appears similar functionality will be provided for riders of US metro systems.
A number of cities in the US have been gearing up for wireless payment services. New York City announced in 2017 that it would be moving away from MetroCards in favor of tap-to-pay turnstiles. Likewise, Los Angeles has been working toward introducing Apple Pay support.