Google Maps adds live transit updates and service alerts for select cities.
Google Maps for mobile and desktop now makes transit arrival times available by clicking on the station icon for transit systems in San Francisco, San Diego, Portland OR, Boston, Madrid, and Turin.
Me, I like to use NextBus, which has much more usable visual interface because they show the actual location of the next bus or train. NextBus provides up to date “next bus” arrival information for nearly six dozen transit systems in 28 states and provinces.
More at Google Lat Long blog: Know when your bus is late with live transit updates in Google Maps.