track vacancy detection system
([Siemens, Track vacancy detection systems, 2013-05-29, Internet](http://www.mobility.siemens.com/mobility/global/en/urban-mobility/rail-solutions/rail-automation/signaling-components/track-vacancy-detection-systems/pages/track-vacancy-detection-systems.aspx))
Track vacancy detection systems detect and signal whether line sections are clear or occupied, thereby creating the information basis for controlled track occupancy and safe railway operation.
USG: generic term used for track clear detection systems consisting of axle counters as well as for those consisting of track circuits.