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Signal types. North American signals are commonly of three types. Absolute – Absolute signals are usually connected to an interlocking controlled by a block operator or train dispatcher. Their most restrictive aspect is "stop" and trains cannot pass them at stop unless they obtain special authority.
North American railroad signaling. Standards for North American railroad signaling in the United States are issued by the Association of American Railroads (AAR), which is a trade association of the railroads of Canada, the US, and Mexico. Their system is loosely based on practices developed in the United Kingdom during the early years of ...
Centralized traffic control. Centralized traffic control (CTC) is a form of railway signalling that originated in North America. CTC consolidates train routing decisions that were previously carried out by local signal operators or the train crews themselves. The system consists of a centralized train dispatcher 's office that controls railroad ...
A Magnetic flagman wigwag signal in use in southern Oregon, June 2007. Wigwag is a nickname for a type of railroad grade crossing signal once common in North America, referring to its pendulum -like motion that signaled a train's approach. The device is generally credited to Albert Hunt, a mechanical engineer at Southern California 's Pacific ...
Railway semaphore signal is one of the earliest forms of fixed railway signals. This semaphore system involves signals that display their different indications to train drivers by changing the angle of inclination of a pivoted 'arm'. Semaphore signals were patented in the early 1840s by Joseph James Stevens, and soon became the most widely used ...
Pulse code cab signals work by sending metered pulses along an existing AC track circuit operating at a chosen carrier frequency. The pulses are detected via induction by a sensor hanging a few inches above the rail before the leading set of wheels. The codes are measured in pulses per minute and for the 4-aspect PRR system are set at 180 ppm ...
Hall Signal Company. The Hall Signal Company was an American manufacturer of railway signaling equipment in the 19th and 20th centuries. Hall's equipment was widely used by American railroad companies. The company's founder, William Phillips Hall, was an inventor who developed several important devices in the history of railway signalling.
A single lamp attached to wayside signals with a "C" plate bolted to it. The aspect is Rule 280a - Clear to Next Interlocking. This aspect is only seen in the Eastern United States on rail lines operating Cab Signal Systems. Cab signal lines only have wayside signals at interlockings and diamonds.