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.
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
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.
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
Up until the early 1900's, white was commonly used instead of
green for clear, and
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
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
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
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
following terms are taken from a
and a Conrail signal
card. There may be additional differences on other railroads.
1) Normal Speed: The maximum
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
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
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
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
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
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
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
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
for the Kentucky region which appears to use route signaling instead of speed
signaling, another is the
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.
Every signal aspect nowadays has a rulebook number associated with it.
For instance, The Baltimore Light Rail
manual shows us that a stop signal is rule number 4.4.1.
the restricting signal above was on the Santa Fe (which this one is not), it
would be rule number 240 in one
(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
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 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
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
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
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
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
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
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
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
Beginning in 1912, every country approved of and received
designated letters to begin radio station call letters with.
In the United States, the letters "W" and "K" were to be used.
At first, it didn't matter what part of the country a station
was located in to use either letter. Then, in 1923, The
Federal Communications Commission ordained that all new radio
stations east of the Mississippi River would use "W" as the
first letter and stations west of the Mississippi would use "K".
Certain stations were "grandfathered" and allowed to keep their
call letters for various reasons, even if they did not conform
to the new edict. By the way, Canadian stations begin with
"C" and Mexican stations begin with "X". More at:
Immediate speed / Occupancy / Next
signal rules Speeds determined by frog
angles Cab signal require acknowledging more restrictive signal or brakes
apply. Semaphore-Blade shape and color have no affect on