North American railroad signals generally fall into the category of multi-headed electrically lit units displaying speed-based or weak route signaling. Signals may be of the searchlight, color light, position light, or color position light types, each displaying a variety of aspects which inform the locomotive operator of track conditions so that they may keep their train under control and able to stop short of any obstruction or dangerous condition.
There is no national standard or system for railroad signaling in North America. Individual railroad corporations are free to devise their own signaling systems as long as they uphold some basic regulated safety requirements. Due to the wave of mergers that have occurred since the 1960s it is not uncommon to see a single railroad operating many different types of signaling inherited from predecessor railroads. This variety can range from simple differences of hardware to completely different rules and aspects. While there has been some recent standardization within railroads in terms of hardware and rules, diversity remains the norm.
This article will explain some of the aspects typically found in North American railroad signaling. For a more technical look at how signals actually work, see North American railway signaling.
Signaling aspect systems There are two main types of signaling aspect systems found in North America, speed signaling and weak route signaling. Speed signaling transmits information regarding how fast the train is permitted to be going in the upcoming segment of track; weak route signaling transmits information related to the route a train will be taking through a junction, and it is incumbent upon the engineer to govern the train's speed accordingly. Weak route signaling is applied with the term weak because some speed signal aspects may be used in the system and also because exact route information is not typically conveyed, only the fact of a diverging or straight route, each having a predictable range of known speeds.
Typically railroads in the eastern United States ran speed signaling, while railroads in the west used route signaling, with some mixing of systems in the Midwest and South. This was due to the lower train density in the west combined with generally simpler track layouts. Over time, the route signaling railroads have incorporated segments of speed signaling through merger and have also adopted more speed-based aspects into their systems. Of the five major Class 1 railroads in the United States, CSX uses speed signaling, Union Pacific and BNSF use speed enhanced route signaling (or what by now is effectively speed signaling with some route elements), and Norfolk Southern uses a mix of speed and route signaling based on the original owner of the line. Commuter railroads and Amtrak all use speed signaling where they own or maintain the tracks they run on. Canadian railways all use a strong system of speed signalling in Canada, but have some segments of route signalling on lines they have acquired in the United States.
Common signaling practices
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. Absolute signals will default to displaying stop unless expressly cleared by a control authority. In older practice, multiple signal heads are directly above and below each other on the mast. Automatic Automatic signals are governed by logic connected through electrical track circuits which detect the presence of trains or obstructions automatically. Automatic signals are permissive with their most restrictive aspect being one of the "restricted proceed" variety. Trains can pass an automatic signal displaying "restricted proceed" without any outside permission. Automatic signals are typically recognized by having an attached number plate and in older practice, having multiple signal heads offset from each other on the mast (i.e., on opposite sides of the mast). Semi-automatic Semi-automatic signals are those that typically act as an automatic signal, but can be set to display an absolute "stop" aspect. Semi-automatic signals do not have a number plate, but can display an explicit "restricted proceed"-type signal.
Other types of signals include train order signals, manual block signals or signals governing special safety appliances such as slide fences, non-interlocked sidings, road crossings, etc. These are much less common than the three standard types.
Layout
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. The signal's aspect is based on a combination of the aspects each individual head displays. Where a signal has multiple heads, aspects are read from top to bottom and are described as "X over Y over Z".
Dwarf signals are smaller signals used in low-speed or restricted-clearance areas. Most signaling aspect systems have a parallel set of aspects for use with dwarf signals that differ from aspects used in high signals. Dwarf signals may have multiple heads just like a high signal, but sometimes dwarf signals use so-called "virtual heads" to save on space and cost. This is where a dwarf signal displays multiple lamps on what would ordinarily be a single signal head creating the effect of multiple signal heads. For example, a stack of dwarf lamps in the order yellow, red, green can display plain yellow, red and green as well as yellow over green and red over green.
Behind the signal head is placed a dark backing or target, which helps improve signal visibility in bright ambient lighting. Target designs vary, but are usually round or oval, depending on the layout of the signal lamps. For each type of signal there are usually a range of target dimensions that can be chosen by the individual railroad company. As dwarf signals are not designed to be seen from long distances, they are not generally equipped with targets.
Mounting
Signals are most commonly mounted on trackside masts about high to put them in the eyeline of the engineer. Signals can also be mounted on signal bridges or cantilever masts spanning multiple tracks. Signal bridges and masts typically provide at least of clearance over the top of the rail. Bracket masts are arranged with multiple signals are mounted on the same masts governing two adjacent tracks. Bracket masts tend to be the tallest type of signal to allow the train crew to see the signal over a train on the intervening track. Signals in electrified territory may be mounted on the catenary structure, and signals on bi-directional lines may be mounted back-to-back on the same mounting device.
Prior to 1985, signals were required by regulation to be mounted above and to the right of the track they governed. This mounting was designed to allow the engineer to view the signal when driving a steam or diesel locomotive with a long hood that restricted the view to the left. In most situations, especially where bi-directional running was implemented, signals needed to be mounted above the track or on bracket masts to allow this right hand placement. As locomotive design changed to allow good visibility on both sides of the track, regulations were changed allowing railroads to shift to bi-directional mast type signals, using signal bridges only in special situations involving multiple tracks or restricted views.
Dwarf signals are typically mounted on the ground in areas of low speed movements or restricted clearances. Dwarf signals may be sometimes mounted higher up on a small mast or other structure for improved visibility. These can be known as "high dwarfs" or "stick signals," but a tall mounting does not change the lower speed applications of the dwarf signal.
Signal colors and lamps Electric signal lamps are typically low-power (35 watt) incandescent lamps running on low-voltage direct current or, more recently, high-output LED arrays. Incandescent signals use a doublet lens combination to directionally focus their small power out over a long range ( in daylight). New LED signals may either use an unfocused array or act as a drop-in replacement behind a traditional lens. U.S. signal lenses have a standard diameter of . North American signals use a standard set of colors, defined in October 1905, and which became common to other modes of transportation as shown on page 384 of the Simmons-Boardman 1911 Signal Dictionary.
Green – Used to indicate "clear" or proceed. Yellow – Used to warn the engineer of an impending stop or speed reduction for an occupied block ahead. Also used for low-speed movements. Red – Used to indicate a full stop or other restrictive condition, or used as a placeholder light (when that part of a signal is unused but to confirm to the crew the signal is working, so as not to require guessing the rest of the combination in case of a light failure). Blue – When on a signal doll post, indicates intervening track between the signal and the track to which the signal applies, or to indicate all equipment on the section of track to the rear of the blue signal is not to be moved as workers are under, on, or in the equipment. Purple – Obsolete. Up to around 1940, purple lenses were used rather than red as the "Stop" indication in some yards and derails. In 1952, the Interstate Commerce Commission ruled that purple should no longer be used in the U.S. for that purpose. Lunar white – Blue-filtered light to eliminate all trace of yellow used to indicate a restricted proceed condition. Lemon yellow (the AAR's official name) – Used in position light systems as an all-purpose high visibility color, greatest fog penetration. (Plain) white – Plain incandescent white light. Used in dwarf position light signals with frosted lenses.
Individual signal heads may be set to flash a color to create a different signal aspect. Signals in the United States typically flash only one head at a time, while signals in Canada may flash two heads at a time; flashing lights are generally less restrictive than steady lights.
A few rapid transit systems utilize just two signal lamp colors (lunar white for proceeding and red for a full stop); examples include the Baltimore Metro SubwayLink, the Washington Metro and the PATCO Speedline.
Speeds Signal rules and aspects make use of several pre-defined speeds. These speeds are also used in Weak Route type signaling.
Normal speed – The normal speed for the railroad line, also known as maximum authorized speed (MAS). Limited speed – A speed less than normal speed that was employed starting in the 1940s for use with higher-speed turnouts (switches). This speed is defined by individual railroads and ranges from to . Medium speed – Original concept for a standard "reduced" speed normally set to and can range as high as . This is the typical speed for diverging movements through interlockings and is also the speed trains are limited to when approaching a Stop or Restricted Proceed-type signals. Slow speed – while within the limits of an interlocking and 20 mph when not in the limits of an interlocking. This is used for trains negotiating complex trackwork at interlockings. Restricted speed – Used for trains entering or operating in unsignaled territory or when entering a de-energized track circuit. Regulatory definition of no greater than outside interlocking limits, 15 mph within interlocking limits. Trains operating at restricted speed must be able to stop within half the range of vision short of any obstruction, and must look out for broken rails.
Fault tolerance