Atlanta's public transit system (MARTA) has an app that shows real-time location of buses, but doesn't currently show locations of trains. Estimated time of arrivals are only shown on LED screens inside train stations. Unlike other forward-thinking cities' public transit authorities, MARTA currently does not have a public API to access their data. Thus, the team who wishes to tackle this problem will need to do some community-organizing to encourage MARTA's board of directors to open up the data.
As a subway rider, I would like to know how quickly I need to jog/walk to catch the train by checking a mobile app that shows the real-time train location and expected time of arrival.