Enabled by default, the alerts pop up in the Waze app when a driver is approaching a railroad crossing on a street. The feature can be disabled in Waze’s in-app settings under Map display > Reports > Railroad crossing > Alert me while driving.
Waze is available for free on the App Store for iPhone and iPad.
In related news, Google Maps will be returning to the Apple Watch in the coming weeks. The app supports estimated arrival times and step-by-step directions on the Apple Watch when using saved destinations like Work or Home. Other destinations require navigation to start on the iPhone, with the directions then picked up by the Apple Watch.