Inside the MBTA Operations Control Center (OCC) Courtesy: MBTA
With last weeks unveiling of the MBTA's new "Next Train" signs for the Green Line, some of you might be wondering why does the Green Line still not have real time arrival data?
Map of AVI locations Courtesy: MIT
Although these new "Next Train" signs are new to the Green Line, the technology that they run on has been around for the past two decades. AVI or automated vehicle indicators are how the Green Line is currently tracked by MBTA employees.
Around the system there are a series of AVI readers embedded in the tracks. When a train passes over one of these readers it reports the trains location to the T's Operations Control Center (OCC) which is located at 45 High St in Boston. When a train passes a specific point it's number appears next to that AVI point on a large map at OCC. This means the T knows that a specific train is at or beyond that specific point. When it reaches the next reader it will disappear from the first point and appear on the next one.
Seems like the T should know where most Green Line trains are right? Unfortunately, it's not that easy. In the central subway, which runs between North Station and Copley Station, the AVI points are somewhat close to one another. When the trains go above ground however, the points are very far apart. All above ground routes are monitored by an actual human being. An MBTA inspector will post themselves at a certain stop along the line and monitor when trains are passing by.
Still a little confused? Here's an example: Lets say Green Line train 3651, which has a destination of Boston College, just passed the AVI point at Copley station. The T knows that the train is at or beyond Copley but it has not yet passed the next point which is at Kenmore Station. Once the that train passes Kenmore the next AVI point isn't until Chestnut Hill Ave. which is 16 stops from Kenmore. On average, that's about a 25 min ride over 3.5 miles. During that time central control has no visual on that train unless the inspector on the line physically reports its position.
Overall, the T knows what train will be next (B,C,D,E) but they do not know the spacing between trains. Therefore no real time train data can be determined for the Green Line. These new "Next Train" signs are a big step for the Green Line but bigger and better things are coming. Back in January the MBTA announced that they would have real time tracking data for the Green Line by 2015. Since it's still over a year away, these "next train" signs will have to do for now.