Vital track circuit wheel detector

A vital, or fail-safe, track circuit wheel detector for detecting the presence and position in a given detection zone of wheel-axles on railroad cars; in order to provide the requisite fail-safe indication, the operation of the system relies on a sensing means which produces two distinct output signals. The sensing means includes a biasing arrangement for providing, even in the absence of wheel-axles, a steady output signal derived from the path of normal feed current flow. This bias signal is countered or opposed by a wheel sensing signal from the same sensing means responsive to flux level changes resulting from the presence of particular directional components of wheel-axle shunt current, whereby a dip or net reduced signal is effectuated; in the case of open circuit conditions, the output indication goes to zero when there are no wheel-axles in the detection zone.