Cab signalling is a railway safety system that communicates track status and condition information to the cab, crew compartment or drivers compartment of a locomotive, railcar or multiple unit. The information is continually updated giving an easy to display to the train driver or engine driver. The simplest systems display the signal, while more sophisticated systems also display allowable speed, location of nearby trains. The main purpose of a system is to enforce a safe separation between trains and to stop or slow trains in advance of a restrictive situation. The first such systems were installed on a basis in the 1910s in the United Kingdom, 1920s in the United States. In North America, the track circuit system developed by the Pennsylvania Railroad. All cab signalling systems must have a continuous in-cab indication to inform the driver of track ahead, however. Intermittent cab signals are updated at discrete points along the rail line, continuous cab signals receive a continuous flow of information about the state of the track ahead and can have the cab indication change at any time to reflect any updates. The majority of cab signalling systems, including those that use coded track circuits, are continuous, the British Automatic Warning System, German Indusi, and Dutch ATB-NG fall into this category. Failure to acknowledge the tone will result in brake application, but even after acknowledgement, continuous systems have the added benefit of fail safe behaviour in the event a train stops receiving the continuous event relied upon by the cab signalling system. Early systems use the rails or loop conductors laid along the track to provide communication between wayside signal systems and the train. Cab signals require a means of transmitting information from wayside to train, there are a few main methods to accomplish this information transfer, Electric/magnetic Inductive Coded track circuits. Transponder Wireless This is popular for early intermittent systems that used the presence of a field or electric current to designate a hazardous condition. The British Rail Automatic Warning System is an example of a cab signal system transmitting information using a magnetic field. Inductive systems are systems that rely on more than the simple presence or absence of a magnetic field to transmit a message. Inductive systems typically require a beacon or a loop to be installed at every signal. The inductive coil uses a magnetic field to transmit messages to the train. Typically, the frequency of pulses in the coil are assigned different meanings
Cab signal display unit on a Chicago Transit Authority'L' train. The vertical light bar in the middle of the signal indicates the maximum permitted speed for the section of track where the train is currently located.
Two-aspect AWS indicator showing caution. All black means clear.
CDU used on Metro-North is integrated with the speedometer indicating which speed limit applies for each signal.
Amtrak ACSES-capable cab signal display unit showing both a miniature signal and associated speed limit.