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CHAP 1 - Introduction
CHAP 2 - Some Basic Specs
CHAP 3 - Glossary of Signal Terminology
CHAP 4 - Reading And Interpreting Railroad Signals 1
CHAP 5 - Reading And Interpreting Railroad Signals 2

Let's go over the topics for discussion on this and the following page.  Once you get done reading these two pages, you should have a fairly good grasp of the nature of railroad signals.  As with any discussion of a topic that covers so many railroads, and so many practices, the information presented here is a guide and may not be correct for the railroad you are interested in.

Name, Aspect, Indication   How do we properly refer to signals.

Signal Colors and Their Meaning   Every color tells you something, do you know what they all are?

Flashing Aspects   The meaning of a signal changes if the signal is flashing, we'll tell you why.

Speeds   Besides telling the engineer whether he needs to stop or go, we also need to tell him how fast he can go.

Route vs. Speed Signaling   Railroads have a choice when defining the signal indications.

Rulebooks and the Signal Rules   Every indication of a signal corresponds to a rule somewhere....

the Signal Head   What exactly is a signal head?

High vs. Low (or Dwarf) signal   The same signal might mean something different if mounted low, if it even looks like the high version!

Format   Is the red on the bottom or the top?

The second page of this section will deal with the signals, modification of them, and how it affects the signal indication.

Multiple Heads   Why do we need more than one signal head, and what do they all mean?

Red as Place Holder   A Simple Concept.

Darkened Heads   Why the railroads would selectively turn off a signal head.

Stop vs. Stop & Proceed   Why the railroads came up with this indication and it's impact on operations.

Stop Signals vs. Grade Signs   Another way a railroad can help to control costs.

Blanking out unused positions   A simple way to control your inventory.

Signal Spacing   How it changes the meaning of a signals indication.

Signal Bridges   When you have more than one track to signal.

Doll Posts   A placeholder for un-signaled tracks.

Backgrounds   Helping with the visibility of a signal.

For a very nice interactive signal display of the NORAC rules, check out this page

Name, Aspect, Indication???



Hope I don't bore you, for I will be repeating some of the stuff I already mentioned in sections one and two.....

I guess the first thing you need to know about a signal is: what is the signal telling you?.... what information is the signal trying to convey to the engineer (railroads) or operator (transit).  To intelligently discuss a signal, you need to be able to differentiate between a signals aspect, indication, and name (and maybe the rule it is associated with).


Name - The name of a signal is just that, what the signal is called when you look in a rule book or talk about it to others.  The above signal is called "stop signal", and the one to the left is "restricting".

Aspect - The aspect of a signal is the visual appearance of a lit signal, for instance, in the above photo, the signal is displaying a red.  As shown to the left in a two head signal, you have a lunar over a red.  

Indication - The indication of a signal is the meaning of a signal.  The red signal above tells the light rail operator he needs to stop before passing the signal.  The signal to the left tells the engineer that he can not pass the signal at more than (usually) 15MPH - if he is exceeding that speed, he needs to slow down to that speed before getting to that signal.

Signal Colors and Their Meaning

The colors used in today's signals are green, yellow, red, lunar, and white (remember, we're talking about signals in North America).

The color of a signal tells the engineer what he has to do, the location of that color, determines the speed at which he is allowed to "do it".  We will get to the speed thing in a minute.

We are familiar with red, yellow, and green, for those colors are used in the traffic lights that govern our every move on the roads around us.  On the railroads, these colors are used to display stop, approach, and clear, respectively.  Lunar is a relative newcomer to most signals, and probably saw it's first widespread use in B&O CPL signals for the restrict indication.  White today is used in CPL marker lamps, as well as Pennsy dwarf and pedestal signals.

Up until the early 1900's, white was commonly used instead of green for clear, and purple was common in addition to red for stop and/or restrict, but mostly in dwarf signals.  My signal timeline has more details of signal development and progress.

In general, the four main colors and their indications are:

Green / Clear:  The train may proceed at the maximum allowable speed in the rulebook for that stretch of track, until it reaches the next signal.  The use of green in signals had to wait for several developments back in the early 1900's.  One was the development of a suitable coloring agent for glass to give it the color, and secondly, the railroads needed a push to stop using white for clear, since it was not failsafe... in other words, if a yellow or red lens was broken, and fell out, it would display white, and the engineer would interpret that as clear.

Yellow / Approach:  The train must proceed at an intermediate speed, usually medium speed, until it reaches the next signal.  Not all railroads employed yellow for the approach indication.  The CNW back in the early days used a combination of red and green to indicate approach, as shown in the CNW rulebook section.  The use of green and red wasn't limited to railroad use, as many municipalities did the same with traffic lights, and I can remember traffic lights in New York City into the early 60's using red and green lit together to warn of the signal getting ready to change to red.

Lunar /Restricting:  The train must proceed at the restricting speed until it reaches the next signal.  Lunar is a very slightly bluish white, and it compared to a "regular" white, has a distinct difference in it's appearance.  The lens itself is a pale blue, and if used to filter natural sunlight, it would give off a bluish color.  What makes the resultant color in a signal look white is the fact the an incandescent bulb gives off a lot of red, and little blue, so the filtering effect of the lens corrects the light passing through to give it its cool white look.  Restricting was a solution to a situation for the railroads, where they were trying to get around the practice of having to stop a train, which costs them a lot of money.  Proceeding at a restricted speed allowed the train to creep along while keeping a look out for anything that may cause an accident.  Railroads also came up with a stop and proceed indication, which at least allowed the train to keep moving after coming to a complete stop, and probably saving a little bit of time.  Restricting speeds are usually around 15MPH, but can be anything from 10 to 20MPH, depending on the railroad.

Red / Stop:  Trains must stop for a red signal.  Some railroads employ both "stop and stay" and "stop" indications, the latter of which allows the train to proceed at restricting speed after the engineer has stopped his train at the signal.  On multiple head signals, red is also used as a placeholder.

The pictures below illustrate these four aspects.  Not many signal heads contain all four aspects as this one does in Doswell VA.


All four of these aspects can be displayed on color light, position light (PRR), color position light (B&O), and position color light (Amtrak) signals.  Searchlight signals and "tri-light" signals can only display three of the aspects by nature of their design.

Flashing Aspects

Because of the definition of aspect (it's visual appearance), we can gain an additional aspect from any of the four above by flashing the lamp on and off.  In most cases, this upgrades, or loosens the control the signal has over the train movement.  In the picture below, from the ATSF section, one can see that if we flash the red lamp, it changes a stop signal to a restricting signal, and allows the train to proceed at that speed without stopping.  Many railroads would accomplish the same thing by using a number plate. 

In the second example, CSX uses a flashing aspect to upgrade a medium-approach-slow (R/Y/G), to a medium-approach-medium by flashing the green aspect.  The yellow in the middle gives us the medium approach, using medium speed through the turnout, and the flashing upgrades the slow speed to medium speed on the approach to the next signal.  While it may sound complicated, it is actually pretty simple and follows the rules.


There are a number of "speed" terms associated with signals and the rules.  Some speed terms are associated with just signals, and others are used in instructions by the dispatcher 

The following terms are taken from a Southern Rwy rulebook and a Conrail signal card.  There may be additional differences on other railroads.

1) Normal Speed:  The maximum authorized speed.

2) Limited Speed:  A speed not exceeding 45 miles per hour.  Some railroads, such as the New York Central (aka: CR) specified 45MPH for passenger trains, and 40MPH for freight trains.

3) Medium Speed:  A speed not exceeding 30 miles per hour.

4) Reduced Speed:  Proceed prepared to comply with flagging signals and stop short of train or obstruction.

5) Restricted Speed:  Proceed prepared to stop short of another train, obstruction, or switch not properly lined and look out for broken rail, but at a speed not exceeding 15 miles per hour.  Conrail specified 20MPH outside interlocking limits, and 15MPH inside.

6) Slow Speed:  A speed not exceeding 15 miles per hour.  Some railroads have pushed this up to 20MPH, such as CSX.

7) Yard Speed:  A speed that will permit stopping within one-half the range of vision.

Of the above speeds, several have associations with signal indications.

  Normal speed is associated with a clear indication, whether it be a lone Green on a high signal, a G/R, or G/R/R.

  Medium speed is associated with a green indication in the middle position, medium clear, as in a signal displaying R/G/R.

  Move the green down to the bottom position of a three head signal, R/R/G, and we have slow speed, as in slow clear.   

Maximum track speeds are usually specified in the employee timetable.

In most rulebooks, they will state that a train passing a signal has to maintain the speed indicated by that signal until the rear of the train has passed that signal.  The same generally applies for moves through turnouts (switches).

Route vs. Speed Signaling

Not all railroads use their signals to indicate the speed of the train.  Generally, the western roads following Route Signaling, and the eastern railroads use speed signaling... It's sort of like the divide between radio using "K's" in the west, and "W's" in the east, except KDKA in Pittsburgh (there's always an exception!)(see the bottom of the page).

The first example below is from an ATSF rulebook, and illustrates the use of routing information instead of speed information for the indications.  Rule 237, diverging clear, would be a medium clear on the east coast, and the diverging approach in the east would be either a slow approach or restricting.

Exceptions abound.  One is the NS timetable for the Kentucky region which appears to use route signaling instead of speed signaling, another is the Southern.

But even with railroads that do use route signaling, sometimes the indications can give speed info as seen in an excerpt from the same ATSF rulebook:

In the ATSF rulebook, it does not show any three head signals.  There has been much discussion on the Yahoo Railway Signaling group about the differences in speed and route signaling, and some roads do employ three heads.  In this case, they would use the middle head to indicate a diverging route one way, and the lower head would be for an alternate or immediately following diverging route.

Rulebooks and the Signal Rules

Every signal aspect nowadays has a rulebook number associated with it. 

For instance, The Baltimore Light Rail system operators manual shows us that a stop signal is rule number 4.4.1. 

If the restricting signal above was on the Santa Fe (which this one is not), it would be rule number 240 in one book (SF's restricting is red over lunar).   Many west coast railroads number their signal rules in the 9.1.x section of their rulebooks, but the numbers are not standardized as the east coast rules are for some reason... for instance, CNW "stop" = 9.1.1, CC&P "stop" = 9.1.9, BN's "stop" is 9.1.15, and another ATSF Rulebook has "stop" = 9.62.

Note that on most east coast railroads,  they number their rules 281 (for "clear"), to 292, for "stop".

My rulebook section is here, and if you have one available you can scan that I don't have, it would be greatly appreciated.  Most of the western roads I do not have, so roads like the UP, SP, WP, etc, would be nice.  Credit is always given.

the Signal Head

The term head has been used a number of times, so what is a head?  Loosely defined, a signal head is a single housing that contains one or more signal elements in it.  Newer signal heads are modularized, allowing the railroads to configure a signal with as many or as few aspects in it that they want.  Older signal heads were a single piece affair, with partitions separating each section (they are also really, really heavy!).  In the above photo, the lunar over red (L/R) signal is a two-head signal, while the red over red over red (R/R/R) signal to the right is a three head signal.  The left signal head below is a three aspect, modularized "high" signal manufactured by Safetran.  Next is a two aspect single housing dwarf signal made by US&S.  Third is a two aspect modularized dwarf signal manufactured by Safetran.  Lastly is a four aspect modularized signal.  All four of these are vertical single head color light signals.


High vs. Low (or Dwarf) Signals

The picture below on the left shows a typical "high" signal installation, this one being in Mason City IA on the UP where it crosses the Iowa Traction.  High signals typically can give authorization for the train to proceed at the maximum allowable speed as given on the timetable or in the rulebooks.

In the middle is a two color, color light dwarf in Doswell VA.  Dwarf signals are usually limited to medium or slow movements.  There are exceptions to this though, since B&O dwarf CPL's can display ALL signal aspects .  In another instance, in Toronto, they utilize three searchlight dwarfs in one housing coming out of Union Station as seen in the second set, which means a G/R/R allows the engineer to "hammer down" coming out of the station.  This is unusual, because in most terminal areas, speeds are limited.

According to one source, a high signal is one which is presented above the level of the locomotive cab window, and a dwarf signal is one presented below the cab window.  This was explained to him by a manager of signals for a class one railroad.

There are some instances where a dwarf signal has been used as a high signal, the only ones I am aware of currently is coming into and out of DC's Union Station, where they employ B&O dwarf CPL's on a signal bridge.  FYI - The B&O color position light signal system is the only system devised that can display all of the high signals aspects in a dwarf signal (the only dwarf signal with that distinction).

The signal to the right (in Perryville MD), as far as I know, is in it's own category, since it is neither a high or dwarf signal.  Most Pennsy pedestal signals are about head height as one walks next to the signal.




These are guidelines, as there are almost always exceptions.  For instance, in Houston at Pierce Junction, the UP mounted a searchlight signal (left photo above) on top of (about) a 6 or 7 foot tall pole.  It is still probably considered a dwarf signal since it is not "way" up.

The picture in the middle is of a Pennsy dwarf PL on a pole in Perryville, and it is mounted on the pole to give the engineer a better sight line, as there is quite a curve on the wye, with a lot of "stuff" in the way.

On the right are the pedestal signals in Wilmington, mounted on a signal bridge over the mainline.  High or not?  After a discussion on the Yahoo Railway Signaling group, most people agreed that it is a medium speed signal since it is almost at the north edge of the platform, in interlocking, so trains would be restricted to medium speed moves anyway.  Notice how some lamps have been replaced by LED's and appear lunar white.


These two excerpts from my Pennsy signal chart illustrates that the same aspect can have two different meanings.  If the dwarf was a high signal, it would be a clear block instead of the slow clear that it is.

Below are pictures of another example, where, back in the early 60's, the B&O tried to implement radio control of the signals on a manual block section after a fatal accident occurred.  They put dwarf CPL's up on the top of 10 or 12 foot tall poles, and re-lensed the signal so all of the aspects were yellow, like the Pennsy PL's are all single colored white.  If you have the July 2004 issue of Trains, you can look for the story by Harold E. Meeker starting on page 50.  Photos are used with the kind permission of Mr. Meeker, who is a most interesting fellow to talk to and has a rich railroad history and background, and if you are into coincidences, he lived about 5 miles away from me when we first moved into the Towson area of Baltimore back in 1966 while he was working for the B&O!!  If you go looking for these signals, forget it, as they were removed shortly after the experiment ended.  A bonus photo he sent me, and not in the article, is the one to the right of the equipment used in the test, an application using what we call today "COTS", or, Commercial Off The Shelf.



I've never seen anyone reference a term for what I call the signal format, where a standard vertical color light signal places the red at the bottom of the signal, and I'll probably get cards and letters from scores of people for creating and using the term.  Having the red on the bottom, instead of on the top as we are accustomed to with automobile traffic lights is a carryover from the old semaphore days, where the blade of an upper quadrant semaphore would "drop" from green to red as a failsafe/safety function in the event of a problem, equipment failure, or other mal-function.  Below is a typical four color progression from red/"stop" to green/"clear" after a train has passed in Doswell VA.  I guess one could call this the standard or normal format (additional photos and a railfan guide to Doswell is here ), and having red on top like the light rail signal at the top of the page, would be reverse format.


When it comes to transit systems, they are less standardized than the railroads, as shown at the top of the page (in what I call the reverse format). 

Transit system signal designers seem to take more liberty when designing systems, and there appears to be more variety in transit signals then there exists with railroad signals.  Many heavy rail systems use a two aspect, three lens red and lunar signal, showing a red over red for stop, and a lunar for clear (the lunar is in the middle, and the top and bottom lenses are red).... Baltimore and Washington DC are two systems that use them.  In addition, they both use a flashing lunar to indicate a diversion from the main route, in which case, it's still really a "three" aspect signal (confusing, isn't it?).  The Minneapolis L/R system started out with signals in the Baltimore format, then decided to switch over to the standard railroad format with red at the bottom.... what difference does it make, and to spend all of that money just because some signal guy came from the railroads and wants to see them look like RR signals.... what a waste of money!


The CNW, at one time, used color light signals horizontally, as shown in their 1929 rulebook: Chicago and North Western Rwy 1929
Even within the rulebook, they show (on two head signals), the lower head with red on the both the right and the left, with rule 501AA showing the red on the left - all others show it to the right.

Another form of color light signal, which many call "tri-lights" (professional signal maintainers do not use this term) for the triangular arrangement of the lenses, generally has the red lens placed at the bottom location, and green to the right.  The signal to the left is at Pierce Junction in Houston, and uses a red LED, and is typical of most tri-light signals in use today.  The signal to the right is the rear view of an older GRS signal (without a background) at the Gaithersburg MD train show.


For a complete glossary of railroad and signal terms, taken from a half dozen or so rulebooks, check out the previous chapter: Glossary of Terms

A Couple Of Other Well Written References:

Signal Basics by AA Krug:

Signal Basics by Carsten S. Lundsten:

Railway Technical Web Pages Signal Basics:

A Wikipedia page:

Immediate speed / Occupancy / Next signal
    Speeds determined by frog angles
Cab signal require acknowledging more restrictive signal or brakes apply.
Semaphore-Blade shape and color have no affect on aspect.

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