Both portions of a center tapped track transformer secondary winding, a load resistor, and a balancing resistor are connected into a balanced bridge network, with the load resistor also connected across the track section rails. A first control transformer primary winding is connected in parallel with the load resistor and its rectified secondary output is applied to charge a timing capacitor which biases a first transistor to conduction to energize the track relay. The primary winding of a second control transformer is connected across opposite, voltage balanced terminals of the bridge to produce, when a track shunt unbalances the bridge network, a rectified output which is applied to fire a second transistor. When conducting, this second transistor discharges the capacitor, thus turning off the first transistor to deenergize the track relay. Removal of the rail shunt turns off the second transistor and the capacitor recharges over a predetermined timing period. The first transistor again fires when the capacitor charge exceeds a preset level. Reenergization of the track relay is thus delayed to avoid loss of train detection under erratic rail shunting conditions.
An alternating current source, normally supplying a basic frequency, is connected to the track section rails through a coupling transformer. In series with the source and transformer primary is a fixed resistance while a fixed inductance is connected in series with the transformer secondary and the rail loop which includes a crossbond between the rails at the distant end. The fixed inductance and resistance are so selected that, when basic frequency energy is supplied with the section unoccupied, equal voltage drops occur across the resistor and the transformer primary which reflects the impedance of the rails, secondary winding, and fixed inductor. Measured signals representing these voltage drops are applied to a voltage comparator. When a train shunt changes the impedance reflected into the transformer primary, the comparator varies the source frequency in order to again equalize the measured voltage drops. Another output from the source is applied to an active type band pass filter which is tuned to the basic frequency and normally energizes a track relay to register an unoccupied track section. However, a shifted frequency output is blocked by the band pass filter and the track relay releases to indicate an occupied track section.
A railway signal system for detecting a train approaching a railroad grade crossing, track section or the like transmits in the track a periodically interrupted carrier wave signal the amplitude of which is attenuated by an approaching train effecting a variable shunt across the track, and an automatic gain control receiving a highly selective filtered input regulates the track signal current to provide a wide window or range of track conditions over which the system operates effectively for train detection. A receiver responsive to the received track signal picks up a signal relay unless an approaching train, broken rail, extreme track ballast or system malfunction is detected. Moreover, an automatic pulse-height control varies system sensitivity to detect rapidly approaching trains at long distances.
A railway signal system for detecting a train approaching a railway crossing or track section transmits in the track a periodically interrupted carrier wave signal the amplitude of which is attenuated by an approaching train effecting a variable shunt across the track. A receiver converts the received signal to a DC level with an impressed AC pulse for application through a DC blocking differentiating capacitor to an amplifier, which produces an AC output used to effect pick up of a signal relay unless an approaching train is detected. The system is self-checking and fail-safe using an astable multivibrator to control power supplied to the relay driver, and the system provides for increased sensitivity with increased train proximity. Additional circuits including a broken rail detector power monitor, island amplifier, loss of shunt detector, and disabling circuit for the latter provide added capability for the system.
A reliable and consistent electrical contact path between the rails of a railroad track established by the shunting effect of the wheels and axles of a train is ensured by applying an alternating current signal to the rails at a voltage level sufficient to overcome poor wheel/rail contact and cause a wetting current to flow between the rails. This likewise provides a current path for railroad control and communication signals, such as the island circuit signal of a grade crossing warning system. The effective range of the wetting current signal is limited by providing the signal with a frequency of 10 KHz or higher to inductively isolate the signal source and associated stretch of track from any shunts beyond that stretch. To avoid interference with other types of electronic track equipment, the signal is applied only when required. In grade crossing warning systems, it is controlled in a fail-safe manner.
A reliable and consistent electrical contact path between the rails of a section of railroad track established by the shunting effect of the wheels and axles of a train is ensured by applying time-spaced, short duration electrical pulses to the rails at a voltage level sufficient to overcome poor wheel/rail contact. Each of the pulses is produced by a generator that discharges a capacitor across the rails. The narrow pulse width inductively isolates the pulse generator and associated track section from adjacent stretches of track, and the spacing between pulses further avoids interference with other types of electrical track equipment. A detector responsive to the voltage on the rails disables the pulse generator when the received voltage exceeds a threshold and thus indicates that no train wheels are present in the track section. In a grade crossing warning system, once activated the pulse generator and detector assume the role of the island detection circuit and maintain the crossing warning.
