the Fitbit mobile app
This app shows predictions for MBTA trains and buses around greater Boston. The display shows the next-arriving vehicles and their directions with an estimated arrival time at a stop. If a stop is only scheduled but without a vehicle enroute then the icon will be blank.
Set up your favorite stops by changing the T Tracker app settings within the Fitbit app. First pick any route that serves a stop you're interested in and then fill in the rest of the details and save the stop. Filter out any routes or travel directions that you don't use often to show a more concise list on your wrist.
To switch stops on the watch click the stop name button at the top of the screen to bring up a selection panel.
This app requires a bluetooth connection with a network-connected phone to retrieve the predictions from MassDOT's public data feed.