Railroad signals generally fall into the category of multi-headed electrically lit units displaying speed-based or weak route signaling.
Certain key elements help define a railroad and shape its personality (if a corporation can be thought of in human terms): a crack passenger train, the unique geography or landscape the railroad crosses, and the type of signaling found along the right-of-way. There are two types of signals, permissive and absolute. You can tell if a signal is permissive or absolute by looking at it. The rules vary by railroad, but permissive signals usually have a number plate on the base of the mast on which they're mounted, or the letter I. Absolute signals do not have the number plate or will have the letter A. The location of the signal is also often a clue to its type Block signals are often approach lit to conserve bulbs and electricity. They remain dark until the approach of an oncoming train is detected, and are illuminated only when a train is in the block that the signal faces. There are two types of signals, permissive and absolute. You can tell if a signal is permissive or absolute by looking at it. The rules vary by railroad, but permissive signals usually have a number plate on the base of the mast on which they're mounted, or the letter I. Absolute signals do not have the number plate or will have the letter A. The location of the signal is also often a clue to its type Pre-signals can be located to stop vehicular traffic before the railroad crossing where the clear storage distance (measured between 6 ft. (2 m) from the rail nearest the intersection to the intersection stop line or the normal stopping point on the highway) is 50 ft. (15 m) or less. An advisory speed plate sign indicating the safe approach speed also should be posted with the sign. The AAWS provides motorists with advance warning that a train is approaching the crossing. The beacons are connected to the railroad track circuitry and activated on the approach of a train. FYI, according to Solomon, the automobile traffic light was adapted from the colorlight railroad signal. How They Work Colorlight signals are simple in concept and design. There really isn't that much to them. For each color of a colorlight signal, there is a bulb as a light source.  Lights. Entire crossing signals are sometimes referred to on the railroad as crossing flashers or flashing light signals, probably owing to the fact that before gates were common the essence of the signal was the lights. In the days of 8-inch crossing signal heads, 18-watt incandescent signal lamps were sometimes used on the mast. Since the advent of 12-inch crossing heads, 25-watt bulbs are used. This is true even on the remaining 8-inch heads if the existing circuits are deemed able to handle the additional current. A signal used in the Delhi metro, typical of urban light rail signals. Usually, signals and other equipment (such as track circuits and level crossing equipment), are powered from a low voltage supply. The colour and shape of the arm is commonly varied to show the type of signal and therefore type of indication displayed. A common pattern was to use red, square-ended arms for stop signals and yellow fishtail arms for distant signals. A signal is a mechanical or electrical device erected beside a railway line to pass information relating to the state of the line ahead to train/engine drivers (engineers in the US). The driver interprets the signal's indication and acts accordingly. The colour and shape of the arm is commonly varied to show the type of signal and therefore type of indication displayed. A common pattern was to use red, square-ended arms for stop signals and yellow fishtail arms for distant signals. North American railroad signals generally fall into the category of multi-headed electrically lit units displaying speed-based or weak route signaling. North American signals generally follow a common layout. A high signal consists of one to three heads mounted roughly in a vertical stack, each head capable of displaying one to four different aspects. Automatic signals are identified with a number plate whereas absolute signals are not. Also, the detection circuits may have other outputs to provide train information to controlled highway intersections that are near the crossing. But in any case, the key concept is that when a train is approaching the detection circuits drop the master relay to activate the signals. In the days of 8-inch crossing signal heads, 18-watt incandescent signal lamps were sometimes used on the mast. Since the advent of 12-inch crossing heads, 25-watt bulbs are used. This is true even on the remaining 8-inch heads if the existing circuits are deemed able to handle the additional current.
http://trn.trains.com/Railroad%20Reference/ABCs%20of%20Railroading/2006/05/Railroad%20signals.aspx http://trn.trains.com/Railroad%20Reference/ABCs%20of%20Railroading/2006/05/Railroad%20signals.aspx http://safety.fhwa.dot.gov/xings/com_roaduser/07010/sec04b.htm http://www.railroadsignals.us/signals/color/index.htm http://signaldepartment.com/articles/xing-signals/default.html https://en.wikipedia.org/wiki/Railway_signal https://en.wikipedia.org/wiki/Railway_signal https://en.wikipedia.org/wiki/North_American_railroad_signals http://signaldepartment.com/articles/xing-signals/default.html
what is a Railroad signal unit