The Washington Metropolitan Area Transit Authority (WMATA) has got some explaining to do. According to yet another damning report on MetroRail that appeared in The Washington Post yesterday, the apparently failure-prone ‘fail safe’ Automatic Train Protection (ATP) system is again at the center of the report.
Metro has a two-layer system designed to prevent trains from slamming into one another. First, Automatic Train Control (ATC) usually runs the trains automatically and makes sure that trains don’t enter a ‘block’ (circuit) occupied by another train. If ATC is off and the trains are running manually, or if ATC fails, the secondary ATP system [supposedly] kicks-in and cuts power to a train entering an occupied block.
The problem is that both systems rely on a single, non-redundant network of track circuits. If a track circuit fails, both ‘layers’ of the protection system evaporate into nothingness. This is how the collision happened in June; both layers of the ‘fail-safe’ system got bad data from one bad track circuit and, because there was no independent backup, people died.
