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Railroad Signals
Grade crossing signals were also provided at road crossing to warn motor vehicle drivers of an approaching train. Railway signalling is a system used to control railway traffic safely to prevent trains from colliding. Trains are uniquely susceptible to collision since they frequently operate at speeds that do not enable them to stop quickly or, in some cases, within the engineer's sighting distance. Whistle Signals Standard Whistle signals were developed as a means for the locomotive engineer to indicate to the train crew and others nearby what he was about to do. Common Locomotive Whistle Signals, "O" indicates a short sound and "—" represents a long sound:
When a train crew was picking up or dropping cars off at a siding, or waiting for another train to pass, standard lantern or hand signals would be used by the on the ground train crew to communicate to the engineer and other train crew members.
Block Signals Block signals are used to indicate to a train crew that another train is in the block they are about to enter, or in the following block of track. Initially block signals were manually controlled by a human operator. Before clearing the signal, he was required to verify that any train that had previously entered the block was now clear of if, written records were kept, and a prescribed was procedure followed when communicating with another operator. As the railroads main lines became busier, automatic block signals were introduced. The Southern Pacific completed installation of automatic block signals on their main lines in 1931.
Sometimes there would be multiple signals on one pole to provide separate indications for the mainline and the siding. The lower signal would be for the diverging track. There were a number of indications possible, for a single signal:
Other combinations of multiple signals, and signs on the pole, such as the letter "G" communicated additional information to the locomotive engineer and train crew. Grade Crossing Signals Initially only a painted warning sign was posted at a railroad crossing, commonly this sign was in the form of an X with "railroad crossing" painted on it. As more motor vehicles began to use the roads it became necessary to provide a warning that a train was approaching. Lighted grade crossing signals in various forms emerged, "wig-wag" signals mechanically moved a single red light back and forth, later signals used two lights alternating flashing. In busy city areas mechanical gates would close. This section is still in development, please check back soon.
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Some block signals were "semaphore" flags with a light. Often a semiphore would have an uper and lower flag close
together. Other block signals were just a light by itself. 