Boston's MBTA seems to be leading the way with this kind of information, and with making it open to developers. http://mbta.com/rider_tools/apps/ http://mbta.co

Boston's MBTA seems to be leading the way with this kind of information, and with making it open to developers. http://mbta.com/rider_tools/apps/ http://mbta.com/rider_tools/apps/ lists dozens of web sites and mobile apps which use data about the MBTA's buses and trains. The MBTA's bus data (via http://www.nextbus.com http://www.nextbus.com, the giant in this industry) tends to be pretty good, though sometimes buses disappear from listings and reappear, and sometimes the predictions aren't very accurate, but they're definitely useful. Subway train data is only fed into the system once a train leaves the end of the line, so there tends not to be data for inbound trains for the first several stops. Commuter rail data is still different, because trains are so much less frequent and people tend to aim to be on a particular train which they know based on its scheduled time. Other transit agencies are much more closed about their data. For instance, DC's WMATA requires you to sign up for an API key and access all data using their web services, even though their bus data comes from NextBus so you'd think they could let you hit NextBus's web services for their data. There's also the brand-new GTFS-realtime spec from Google, which is a Protocol Buffer format for getting vehicle positions and so on very efficiently. The idea is that you'd be able to get the current locations of all the vehicles in a large system, like the MBTA, in a single request. (Shameless plug: one of the MBTA apps listed in the app catalog is my web site http://www.mbtainfo.com http://www.mbtainfo.com)