After an accident at Boston's Back Bay Station on December 12, 1990, Amtrak and the FRA reviewed all curves on the Northeast Corridor where trains might derail if the operator failed to comply with the lower speed specified for the curve.[1] Ten curves between Boston and Washington, DC, met that description. On each curve, a cab signal change point was added to drop the cab signal from clear to approach medium, which required the engineer to slow the train to 45 mph to negotiate the curve and enforced that speed if the engineer failed to do so.
There was cab signal protection for westbound trains at the accident location because the maximum approach speed for westbound trains was 110 mph, higher than the overturn speed of 98 mph.[2] Westbound train engineers approaching the curve at the accident site would have received a cab signal warning to reduce the train speed. If an engineer failed to do so, the system would have automatically slowed the train. However, that protection was not added to the accident curve in the eastbound direction because Amtrak assumed that those trains would not be traveling faster than the maximum approach speed of 80 mph. Because the overturn speed was 98 mph, even if an engineer failed to slow from that maximum approach speed, the train would still be below the overturn speed.
The NTSB concludes that cab signal protection to enforce the 50 mph speed restriction in the eastbound direction at Frankford Junction or a fully implemented PTC system would have prevented the accident. Following the accident, Amtrak modified the signal system so that eastbound trains traveling toward Frankford Junction received a cab signal change to ensure speed restriction enforcement until December 2015 when PTC was implemented on that section of track. (PTC enforces speed restrictions, so the signal change was no longer necessary.)[3]
1.5.1 Positive Train Control
PTC is defined in Title 49 United States Code (USC) section 20157(i)(3) as follows:
[A] system designed to prevent train-to-train collisions, over-speed derailments, incursions into established work zone limits, and the movement of a train through a switch left in the wrong position.
PTC uses wireless communication to monitor train movements and automatically stop a train to ensure compliance with speed or signal restrictions thereby preventing collisions and derailments. In contrast, ATC systems rely primarily on hard-wired control points installed in the track and do not provide protection against all overspeed conditions. In other words, PTC is predictive and prevents a signal or overspeed violation; ATC is reactive and waits for a violation to occur before taking some control of the train.
- ↑ National Transportation Safety Board, Derailment and Collision of Amtrak Passenger Train 66 With MBTA Commuter Train 906 at Back Bay Station, Boston, Massachusetts, December 12, 1990, Railroad Accident Report RAR-92/01 (Washington, DC, NTSB, 1992).
- ↑ Overturn speed refers to the speed at which a train would likely derail on a curve.
- ↑ For more information, see section 2.3.
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