Boston’s Massachusetts Bay Transit Authority (MBTA) operates four main subway lines: Red, Orange, Blue, and Green. When we built Visualizing MBTA Data in early 2014, realtime train location data was available for all but the Green Line. On October 23, 2014 the MBTA began publishing this data.

The other lines use a modified freight tracking system with periodic checkpoints to stop a train whenever it gets too close to next train. Since this system also relays checkpoint crossings back to the control center, it provided the infrastructure to track trains early on. The Green Line, however does not use such a system. The trains were not tracked and therefore no location data was available.

The MBTA has been outfitting the Green Line with GPS devices to track the trains while they are above ground and is now publishing this data for close to 98% of the trains to their public API. We collected the first several days of data and show it below in a visualization developed by Étienne-Jules Marey called a “time-space” or “Marey” diagram. Each vertical line represents a station, and time extends from top to bottom. Steeper lines indicate slower trains.

We attempt to shed light on the behavior of the Green Line using this newly released data. While some gaps in data are due to trains without tracking devices, we can still start to see patterns emerge. The Green Line appears to exhibit more consistent volatility than other lines, often due to outbound “clumping” which affects wait times in both directions. This “clumping” originates underground where we can't yet see the trains. The inbound D branch looks most reliable, and outbound B branch looks least reliable. On Friday, late-night T service takes passengers out of the city until 3 am. What else do you notice?