The new Transit feature inside Apple Maps not only has all the scheduling information for the trains and buses in your town, but now it can also alert you to delays.
Incidents and delays—a bus or train running 10 minutes late, a temporary suspension of service—will show up in the Maps interface when you tap on a transit line. And of course, if you tap on a bus stop, you'll see when the next bus arrives.
This eliminates the necessity to download your city's bespoke transit app, or to hit a web service like NextBus—Apple is getting the transit data from the same public source as those services, so you probably should just pick the app or service that works best for your needs. In the case of iOS 9, there's one built into your phone!
