Hierarchy of Cab Signal Systems
With a traditional signalling system, the engine driver must observe wayside signals and act accordingly, depending on the aspects displayed. This method of operation is susceptible to human failure; if the driver does not respond to a signal aspect, a dangerous situation may result. It is also considered "passive," in that the system does not actively prevent the unsafe conditions from arising. Modern cab signalling is an active system, in that the train will default to a safe condition (i.e., brakes applied) if left unattended as it approaches an adverse signal aspect. Thus, the reliance on humans to ensure absolute safety is somewhat reduced.
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