Monday, July 30, 2012

Guest Post: Automatic Stop Announcements, Tracking, & More On The Green Line

Green Line Trolley at BU Central 
     “Entering: Park Street. Change here for the red and orange lines. Doors will open on both sides.” We’ve all heard Frank Oglesby’s mellifluous voice bellowed throughout the interiors of countless green line trolleys. But how does Frank know when to speak up? Or, for the matter, what to say? Surely he can’t just be guessing; he’s far too accurate for that to be the case. So how does he do it? It all has to do with something called Automatic Station Identification or ASI for short. Here’s how it works:
Located approximately 50 meters apart, the green line (similar to the red and blue) has “tags” imbedded in the tracks before, after, and at each station. By themselves, these tags do nothing; they merely have a unique number programmed into them, but require no battery or external power source. Each green line trolley, however, is equipped with a device that actively “reads” the tracks in search of these tags using microwave radio frequencies. When a trolley passes over a tag, the tag number is then communicated to the onboard message system and matched with the appropriate audio and visual messages. At that point, our good friend Frank announces that we are indeed entering Park Street and that one can transfer to the red and orange lines here after exiting the train from either side. With each announcement, the displays also change to reflect what is being said.
While this type of system does work (usually), the one used by the green line is quite dated and therefore its capabilities are rather limited. For example, at the start of each trip the operator must manually enter the trolley’s destination. This not only ensures that the proper audio and visual messages are being communicated to the passengers, but also ensures that the proper track configuration is being used in the tunnels for that specific trip. What’s that mean? It means that, utilizing a system called Automatic Vehicle Identification (AVI), the tracks will automatically align to get your D line trolley to Riverside—not Boston College, Cleveland Circle, or Heath Street—and so on.
Green Line tracks at Chestnut Hill
Another, more major limitation of this system is the fact that most of the information communicated between the track equipment and trolleys is not currently capable of being transmitted back to the MBTA’s Control Center, located at 45 High Street in Boston. This means that not even the T has a good idea of exactly where the next outbound B line (or any) trolley is and when it will be arriving. The only information automatically communicated back to the Control Center is when a trolley has just passed through one of a handful of junctions located throughout the underground portion of the green line. This, however, does not specify the destination of the trolley but merely which direction it is headed (inbound or outbound) giving the MBTA only a very vague idea of where its trolleys are at any given time. Because of this, riders won’t be seeing tracking available on the green line until the MBTA has come up with a way to locate the trolleys themselves.
Aside from its limitations though, the current system does help the MBTA fulfill its federally-mandated ADA requirement for consistent stop announcements and lets commuters and tourists alike know exactly where they’re headed. Here’s to Mr. Oglesby continuing to help passengers find their way for many more years to come.

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Pictures copyright © 2012 Michal Skrzypek


  1. What is a "meter"?


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