Abstract: A system for the transmission of data from a number of out-station transmitters to a central receiver over a communication line or channel in which each out-station transmitter has an associated timer and a detector which "listens" to the line or channel and enables the transmitter only in the absence of detected signals from the other transmitters on the line or channel. When enabled each transmitter transmits data over the line or channel under control of its associated timer for a time interval during which the other transmitters are inhibited. The time intervals commence from the start times which differ from each other by different random delays which are short compared with the timed intervals, so that upon initial switching on of the system one of the transmitters will always commence transmission, followed by all the other transmitters in sequence.

Abstract: An electrical fail-safe filter consists of a transistor amplifier with a frequency-selective positive feedback circuit such as a resistance capacitance network shunting the collector and emitter of the transistor such that failure of any component of the feedback circuit results in a reduction in the overall gain of the stage. The filter is described in its application to a fail-safe receiver for a railway signalling system.

Abstract: Signals such as track signals in a railway signalling system are monitored by antiphase sequential application to an enabling input of a control circuit which starts the operation of a timer, providing an output signal only if the control circuit is enabled for a time interval during which all the signals being monitored are present sequentially. Absence of any one of the signals in this interval results in a fail-safe condition, resetting the timer, which then fails to provide an output. In the application to a railway signal monitoring system absence of any one signal would be indicative of occupation of an associated track section, resulting in a `danger` condition being signalled.

Abstract: A fail-safe timing circuit has a timing capacitor which is charged progressively in steps through a switching arrangement to cause a biasing potential of a threshold device to change progressively. This biasing potential is normally outside the switching range required to cause changeover of the threshold device, and is only brought within this range after a timed interval determined by the capacitor. If the capacitor fails in any sense, the biasing potential fails to reach the switching range and the threshold device is not triggered.

Abstract: In order to detect derailment of a railway vehicle each vehicle in a train is provided with an acceleration responsive device which, in response to the vehicle acceleration resulting from derailment of the vehicle transmits a short range radio signal to a receiver installed in the engine to provide a visual or audio alarm. In one practical arrangement requiring no routine maintenance each transmitter unit has a resonant circuit which is energized as a result of the stressing of a piezo-electric crystal upon vehicle derailment, avoiding the need for any separate electrical power source on the vehicle.