Subway Signals: Single-Line Signal Diagrams
For most of this century, the New York City rapid transit system, under the successive æges of the Board of Transportation, the New York City Transit Authority, and the Metropolitan Transit Authority, has used a consistent and concise formalism to express graphically the layout and capabilities of signals, called the single line diagram, because each track (two running rails and a power rail) is represented by a single line. At its heart a geographically accurate map of a small section of the earth and the railroad, the single line diagram also contains information about the contained signals' nomenclatures, control, and interactions. With its associated notes, the single line diagram provides enough information for a signal engineer to design the relay logic of the signal system for the trackage depicted.
This page explains the notations used in these diagrams, some of which are posted at this site, also used in other explanations on this site. The present page assumes you have some basic understanding of the signal system and the types of signals it involves, seen firsthand and as explained on these pages. For an example of a real set of single line signal diagrams, see the World's Fair Railroad section.
The first thing one is likely to notice when first setting eyes upon a single line diagram is, in fact, the bewildering profusion of lines! In addition to the lines representing the tracks, the single line diagram contains a control line for each signal, representing the extent of track controlled by that signal, which is to say, its control length, that is (definition) the extent of track downtrack from the signal such that if any portion of it be occupied by a train or other danger, the signal will be red (at stop).
Here is a sample single-line diagram of a simple (believe it or not) interlocking, with annotations of the principle elements in red.
The heavy lines are tracks. The thin lines are control lines -- they do not represent physical entities on the right-of-way, but logical distances key to the design of the signals (as explained above). The large single and double circles are signals. The numbers represent the nomenclatures of the signals, switches, and insulated joints, which are sometimes station numbers derived from position along the right-of-way in hundreds of feet, or lever number, which is the number of a lever at the interlocking tower (although there might not be a real "lever", but just a miniature switch or button, this "lever" number is, in effect, a "logical lever" number, and still used to associate equipment with the signal). Each signal symbol consists of heads, which are the familiar boxes (often stacked vertically) with lamps and lenses, and a little "stand" which shows which way the signal faces: the train approaching the signal on the map approaches from the direction with the stand.
Here are the explanations of the symbols for different types of signal heads. The different fill-in patterns convey:
- Whether the signal is automatic or interlocking-controlled (home or approach signals)
- How many and what colors of lenses are in the head, which can possibly be illuminated, not which is illuminated now or at any time.
In this illustration all possible lamps are shown lit at once so you can see what color the lenses are -- in real life, no more than one lamp (other than the paired reds in g and i) is ever illuminated in a single head. The grey circles are "blank lenses" (no lamp, metal cover and no lens).
a and b are used only to represent automatic signals. c, d, e, and f are used to represent the heads of home signals, although c can only appear with itself (twice), or once as a dwarf signal. d and e can appear singly, representing approach signals. f can only be used as the lower head of a home signal offering no choice of route, such as (that controlled as and marked as lever number) 10 in the sample above. The dot h is indicated on the stem of a home signal symbol when the call-on light can be used as part of a "three-yellow" (yard) aspect (although sometimes it is seen on older prints in conjunction with a home signal whose only permissive aspect is a call-on). See Home Signals for a full list of these aspects and their indications (what they mean).
Looking again at the signal layout at our sample interlocking, Let's start with the leftmost signal, with the numbers 2 and F1-240. 2 is the lever number at the interlocking. F1-240 is the station, or "chaining", number, indicating that the signal is 24000 feet along the right-of-way from this line's "zero" (See Joe Korman's list of chaining codes and zeroes for a listing of all letters and zeros.). 2 is an approach signal, because of the solid-filled, single circle representing its single head. The insulated joint at signal F1-240 also shares the nomenclature F1-240. The little tick representing the joint indicates (in this case) that the top-of-the-diagram rail of the track is the signal rail (and is interrupted for insulated joints). The little bird-head symbol pointing down from the track represents the train stop. The fact that it is perpendicular to the track represents it in its "tripping position", which, for this signal, is its canonical state (explained below).
Note that IJ (Insulated Joint) F1-242 has no signal associated with it; it exists solely to allow the control length of signal F1-250 (12) terminate at it. This "cutting" of the section between two signals known as a cut section.
At IJ F1-244, (approach) signal 8 and (home) signal 4 control opposite directions. These are called back-to-back signals: 4, like 2, is for Northbound (moving diagram left to right) trains, and 8 is for Southbound trains. Note that each signal has its own stop, which is beyond the joint in the direction governed by the signal. The lucky signal in the "normal" direction of travel (4) gets to keep the IJ's stationing number. The other ("back-to-back") signal usually gets a stationing number one higher or lower.
The box with the line and dot in it near the switch represents the tower; the dot represents the tower operator, and the line the interlocking machine or panel. Knowing which way the operator faces is crucial, because some signals controlled by some types of interlocking machines are designated L or R according to whether the tower operator moves the controlling lever to his or her left or right to clear them.
The switch bears the number 7; it is controlled by lever #7 in the interlocking tower.
6 is a marker signal, and is never clear. Its stop is shown as up, but it is not always up -- if a route is cleared from 4 to 6, its stop will be driven down. You may be wondering exactly what use is the #6 lever if the signal can never be cleared: in some types of interlockings, such signals receive lever numbers (to associate apparatus with them), and others not. The dotted line around switch 7 is a track-circuit jumper, connecting the two pieces of track around the insulated section of the switch into the same track circuit.
Note that the home signal 10 has an "S" light for the 2-shot grade time control of signal 8 (F1-245) (which is marked as GT).
The open circles (F3-247, F3-249, F1-247) are automatic signals, not controlled at all by the interlocking (well, maybe just a little for F1-247 -- this involves some subtlety and advanced design principles, but in any case, there are no explicit levers controlling that signal).
The notation TRAFFIC LEVER 3 indicates that the direction of traffic on track F1 south of signal 4 is controlled by Lever 3 at this tower, which is tightly interlocked with the controls for all signals here (particularly 8, 10 and 12), as well as with a similar traffic lever at the next interlocking south of here.
The thin lines on the diagram represent the control lines (which encompass the control lengths) of the signals. If any portion of a train, or a switch set the "wrong" (trailing point set to other route) way, or (with some exceptions) the control line of a clear signal in the opposite direction be found in any portion of the control line of a given signal, that given signal must be red. A good fraction of practical signalling can be derived from this definition alone. Looking again at the drawing:
The control line of signal 12 extends southward from the signal along Track F1 to insulated joint F1-242. The start of the control line is labelled with the nomenclature of the signal (12) (Sometimes control lines are labelled only with the track identifier (e.g., F1).) If any part of a train is in any portion of that track, 12 will be red, regardless of its controlling lever (#12). If switch 7 is reverse, not only must the signal be red, but its lever must be "normal" (forcing the signal red). From basic principles of interlocking, we can tell that 7 cannot even be moved reverse unless and until lever 12 is set normal (as a matter of fact, that is why approach signals exist).
The grade-time (GT) signal 8 has a double (as it is "2-shot") "timing section" preceding its control length (that is, starting at F1-250 and going up to the signal itself), indicated by a dotted line. The little circle in the middle separates one "try" section from the other. The timing section is labelled with the nomenclature of the signal it times.
The control lines for signals 2 and 4 have "switches" in them, i.e., fork in a manner identical to the forked track they control. In fact, the control lengths of all signals fork in precisely the same way as the trackage they each control. For example, a (rear of a) train occupying the block between F3-247 and F3-249 will force signal 4 to be red if and only if switch 7 is reverse (set to track F3).
More single-line diagram features
In the diagram above, two more features of single-line diagrams are illustrated. At the bottom of the diagram is the grade diagram, an elevation of the straightened-out right-of-way showing its grade, i.e., its relative flatness or slope, rises and falls. This is important for speed limits and time control. It is sometimes drawn above the main diagram, and sometimes below. The notation 319+75, as often painted on trackside artifacts, signifies 31975 feet along the centerline of the right-of-way from the line's zero. The notation -1.32% indicates a downward (left-to-right) slope of 1.32 percent, that is, the angle whose rise-to-run ratio (tangent) is .0132, that is, 0.756° (0°45'22").
Signal A1-317 is indicated as a Station Time signal, at 20 MPH, by the ST 20 at its symbol. The diamond on the symbol's stem indicates its ability to display an ST Aspect. A1-317's control length extends southward from the signal to A1-321; any portion of train within it will make A1-317 red, unless that portion of a train is only in the dotted portion of the control line, i.e., that shown as a dashed line from A1-320 to A1-321, in which case a train approaching the signal (A1-317) at 20 MPH in its ST Timing Section (indicated by the dot-dash-dot line marked with the signal's nomeclature) can clear the signal. (Note: these timings, distances, and control lengths are not intended to be realistic.)
Note also that the control lines are marked with the station numbers of the signals. Control lines are sometimes marked with track numbers, station numbers, or lever numbers, depending upon the pedigree and intent of the diagram; any is sufficient to disambiguate the control lines.
Stops are show as they are (either up ("tripping") or down ("driven")) when the system is in its canonical (or normal) state. This is defined as the state of the system when all the equipment is operating properly, no trains are anywhere on it, and all levers are in their "normal" position: no signals called, all switches and traffic levers are "normal". In an NX or UR interlocking, no routes are set. All approach and time locking (not yet explained here) is reset. In the canonical state, all stops associated with home or approach signals will be in their "tripping" position, and most stops associated with automatic signals (notably not those associated with GT signals or those in a "reverse" direction of traffic on a given track) will be in their "driven" position. Note that F1-247's stop is normally tripping, because this signal is normally red on account of the approach signal 12.
The canonical state is also important in the wiring diagrams for signalling; the contacts of a given relay are symbolized differently depending upon whether that relay is energized or dropped out when the system is in its canonical state.
Subway Signals: A Complete Guide
Approach, Automatic, and Marker Signals | Train Stops | Time Signals | Interlocking | Home Signals
Sign Signals | Holdout Signals and Bidirectional Traffic | Single-Line Signal Diagrams
NXSYS, Signalling and Interlocking Simulator
Les signaux du New York City Subway
Descriptions and graphics on this page (unless otherwise noted) are Copyright © 1997-2002 Bernard S. Greenberg (contact).