The Federal Railroad Administration (FRA) issued its final rule, including detailed technical specifications, for implementing positive train control (PTC) on all commuter and intercity passenger rail lines, and a significant subset of freight rail corridors.  PTC is an automated, signal-based system of automatically controlling the movement of trains to prevent collisions and the incursion of trains into restricted zones.  The mandate to install PTC appeared in the Rail Safety Improvement Act of 2008, and requires PTC to be installed by December 31, 2015, on each main line over which poison- or toxic-by-inhalation hazardous materials are transported, each main line used for regularly provided intercity or commuter passenger service (subject to certain exceptions), and any additional line of railroad as the FRA may require.  FRA has solicited further comment to address the clarity, certainty, and transparency of the criteria governing relief from the requirement to install PTC.