From: jps@dworkin.wustl.edu (James Sterbenz)
Newsgroups: rec.railroad
Subject: Re: Signal usage (LONG)
Date: 21 Jul 92 16:02:04 GMT
Organization: Washington University,  St Louis MO

In article <1992Jul15.195802.15479@news.uiowa.edu> saevans@icaen.uiowa.edu (Scott A Evans) writes:
>There is probably a FAQ for this somewhere, but I'll ask here anyway.

Actually we need one; this isn't quite it, but here's part of an early
draft on a North American signals.  I hadn't intended it to go out in
this early state, but given the current thread of discussion, here it
is.  I've removed the aspects section since it still needs more work
to make any sense at all to others.  This will be yet another in the
series of signaling througout the world (I've been busy lately
updating the German article to include ex-DDR and Austria)...




                       NORTH AMERICAN SIGNALING
                                
                            James Sterbenz
                       jpgs@watson.vnet.ibm.com
                           +1 314 726 4203
                                
                             21 July 1992
                             version 0.4
                         DRAFT -- DO NOT COPY
                            Copyright 1992



CONTENTS


1. INTRODUCTION

2.    SIGNAL TYPES AND LOCATION
2.1   Signal Aspect Types
2.2.1   Colorlight and Searchlight Signals
2.2.2   Position Light Signals
2.2.3   Color Position Light Signals
2.2   Signal Systems and Location
2.2.1   Manual Block Signals
2.2.2   Interlocking signals
2.2.3   ABS -- Automatic Block Signals
2.2.4   APB -- Automatic Permissive Block Signals
2.2.5   CTC -- Centralised Traffic Control Signals

3.    SIGNAL ASPECTS AND INDICATIONS
3.1   Stop Signals
3.2   Restricting Signals
3.3   Approach Signals
3.4   Clear Signals
3.5   Special Signals
3.6   Switch Point and Track Obstruction Signals
3.7   Hump Signals

4.    FIXED BOARDS AND ADDITIONAL SIGNALS
4.1   Speed Reduction Boards
4.3   Switching Boards
4.4   Grade Crossing Signals/Boards
4.8   Train Marker Lights
4.10  Signals for Train Crews




1. INTRODUCTION
-- ------------


Signals provide 4 positional aspects, and indications for 4 speeds of
diverging routes (plus a restricting speed indication).  Signals are
usually considered speed signals rather than route signals, but this
distinction is rather fuzzy, and depends on the rules in effect for
each railroad.




2. SIGNAL TYPES AND LOCATION
-- ------ ----- --- --------

2.1  Mechanical signals:

North American signals were originally mechanical semaphores.  A few
upper quadrant block and interlocking signals survive, but since their
light aspects are identical to colorlight and searchlight signals,
they are not included in these tables.  The only other significant
remaining installation of semaphores is the Southern Pacific lower
quadrant semaphores.  These consist of a home + distant combination,
with a supplementary lower home arm or searchlight.  Hall effect
"banjo" signals were also used.


Early semaphores:

Semaphore signals wrere originally of the 2 position type, indicating
stop or clear for home signals and approach or clear for distant
signals.  Signals operating in both the upper and lower quadrants were
used.  A carrier with holes containing colored spectacles rotated with
the arm in front of a lamp originally gas, later electric) for the
night indication.

                     |                       V
       r:---       w:|         g:--<       w:|
        :           :           :           :        r/g/w
        :           :           :           :        upper
        :           :           :           :        quadrant
        :           :           :           :

                                              
       r:---       w:\         g:--<       w:\ _
        :           : \         :           : |      r/g/w
        :           :           :           :        lower
        :           :           :           :        quadrant
        :           :           :           :


                     |                       V
       r:---       g:|        gr:--<       g:|
        :           :           :           :        r/gr/g
        :           :           :           :        upper
        :           :           :           :        quadrant
        :           :           :           :

                                              
       r:---       g:\        gr:--<       g:\ _
        :           : \         :           : |      r/gr/g
        :           :           :           :        lower
        :           :           :           :        quadrant
        :           :           :           :


                     |                       V
       r:---       g:|         y:--<       g:|
        :           :           :           :        r/g/y
        :           :           :           :        upper
        :           :           :           :        quadrant
        :           :           :           :

                                              
       r:---       g:\         y:--<       g:\ _
        :           : \         :           : |      r/g/y
        :           :           :           :        lower
        :           :           :           :        quadrant
        :           :           :           :

       stop       clear     approach      clear
            home                   distant

The shapes and colors of semaphore blades varied somewhat across
different railroads and over time, but typical practise is described
here.  Interlocking home signal blades have a square end with a
vertical stripe near the end.  Automatic home signal blades have a
pointed end and a v-shaped stripe to match the point.  Home signal
blades are red with a white stripe.  The night aspect for stop is a
red light.  Originally white was used for the clear aspect but this
was changed to green to avoid the danger of a false clear from a
broken spectacle glass or from surrounding lights.

Distant signal blades are have a notched "fishtail" end and a v-shaped
stripe to match the fishtail.  Originally distant blades were green
with either a white stripe, at night displaying a green light for
approach.

When the change was made to green for clear, yellow was used for
approach and the blades were changed to yellow with a black stripe.
(There was also some use of green and red lamps adjacent for stop, the
blade being green with a red stripe.)

The rear of signal blades is typically white with a black stripe to
match the front side stripe.

There is wide variance in the blade configuration of manual block and
train order; red blades with round ends (and frequently a circular
white stripe to match the round end) or a solid red blade with a
square end were fairly common.

The common combination of a home and distant signal on the same mast
led to three-position signals, with approach indicated by the arm
inclined to 45 degrees in upper quadrant signals.  Lower quadrant
three-position signals were considerable less common, since the
typical design did not allow the arm to drop to completely vertical
for the clear indication.  Originally green was used for approach, and
was later changed to yellow.

                                 |           |
       r:---                   w:|         w:|
        :                       :           :        r/g/w
        :                       :           :V       2-position
       g:--<                   g:--<       w:|       upper
        :                       :           :        quadrant
        :                       :           :

                                  /          |
       r:---                   g:/         w:|
        :                       :           :        r/g/w
        :                       :           :        3-position
        :                       :           :        upper
        :                       :           :        quadrant
        :                       :           :


                                 |           |
       r:---                   g:|         g:|
        :                       :           :        r/y/g
        :                       :           :V       2-position
       y:--<                   y:--<       g:|       upper
        :                       :           :        quadrant
        :                       :           :

                                  /          |
       r:---                   y:/         g:|
        :                       :           :        r/y/g
        :                       :           :        3-position
        :                       :           :        upper
        :                       :           :        quadrant
        :                       :           :

       stop                 approach      clear


Diverging route signals:

At interlockings where diverging routes were possible, multiple
signals were placed on a single mast.  Two arrangements were possible:
The most common arrangement was to place the main (normally straight)
signal highest, with secondary signals below.  In three position
signaling there would be a stack of semaphore, all at horizontal
except for the signal corresponding to the divergence (which would be
at either clear or approach as appropriate based on the block
occupancy of the route set.  In two-position signaling, distant arms
would be located below each home arm.

An alternative arrangement which only gained limited use (but became the
standard in British signalling) was to organise the mast as brackets...



Disc or banjo signals:

Another early mechanical signal that was not as widely used as the
semaphore is the disc, banjo, or Hall signal.  It consists of a large
circular casing that held a colored disc that either obscured a large
circular window, or was rotated to the side showing mostly the white
background.  A small lamp with a colored spectacle carrier was
positioned above the disc opening for the night indication.

        _           _           _           _
      / r \       / w \       / g \       / w \
     | (R) |     | (_) |     | (G) |     | (_) |
      \   /       \   /       \   /       \   /
       |_|         |_|         |_|         |_|
        :           :           :           :        r/g/w
        :           :           :           :

        _           _           _           _
      / r \       / g \       /g r\       / g \
     | (R) |     | (_) |     | (G) |     | (_) |
      \   /       \   /       \   /       \   /
       |_|         |_|         |_|         |_|
        :           :           :           :        r/gr/g
        :           :           :           :

        _           _           _           _
      / r \       / g \       / y \       / g \
     | (R) |     | (_) |     | (Y) |     | (_) |
      \   /       \   /       \   /       \   /
       |_|         |_|         |_|         |_|       r/y/g
        :           :           :           :
        :           :           :           :

       stop       clear     approach      clear
            home                   distant


Light signals:

Modern North American signals are searchlight, colorlight, position
light, or color position light.

Color light designations:

R = red            r = small red          S* = flashing (color S)
Y = yellow         y = small yellow       S/ = dark (lamp off)
G = green          g = small green        S? = on, dark, or absent
U = lunar white
W = white          w = small white
B = blue
V = violet
S = searchlight (multiple colors)




2.1  SIGNAL ASPECT TYPES

2.1.1  Colorlight and Searchlight Signals

These signals are derived from three aspect three speed semaphore
signals, and thus normally consist of up to 3 heads, the upper for
full speed, the middle for medium speed, and the lower for slow speed.
Block signals usually have a single head, but may have multiple heads
in approach to interlockings (which are staggered to clearly
distinguish them).  The most restrictive indication for a block signal
normally is permissive stop (stop and proceed).  Block signals
normally contain a numbered ID plate, which also may be used to
distinguish them from interlocking signals.

Interlocking signals normally have at least two heads, which are
vertically aligned, and indicate an absolute stop (stop and stay).  In
some cases an interlocking signal may have only one head; to indicate
an absolute stop a plate below the signal is commonly added (usually
the letter "A" for absolute), but the absence of a number plate may be
used to indicate the absolute stop.

Each head may may display three aspects: green for clear, yellow for
approach, and red for stop.  Place holders above the allowed speed are
red, below the allowed speed are red or dark, if the head is present.
Thus medium clear is indicated by red over green on a 2 head signal,
and by red over green over red or by red over green over dark on a 3
head signal.  Lunar white is used to display the restricting
indication if necessary.  When the approach to a lower speed is
indicated, the current speed position has a yellow light, the next
speed a green light.  Thus, medium approach slow is displayed by red
(place holder) over yellow (current block speed) over green (next
block speed).

The system gets complicated with the addition of a fourth speed (limited)
and fourth position (advance approach).  These are indicated
by the use of flashing lights, double lights, or both.

Limited speed can be indicated 3 ways.  The original AAR rules
indicate limited speed by 2 green lights, on the middle and lower
head.  A modifier plate is sometimes used on the signal mast, which
indicates that the middle head represents limited and not medium
speed.  The most common method is to flash the middle head when
limited speed is indicated.  Thus, limited clear is red over flashing
green over red, and limited approach is red over flashing yellow over
red.

The fourth position aspect is advance approach, and since another
color is not available, it is indicated by either flashing yellow or
double yellow.  Double yellow has the problem of taking up two speed
positions, and thus only advance approach and medium advance approach
can be indicated.  Flashing yellow solves this problem, but is not
compatible with flashing limited aspects.

The tables of signal aspects (Sec. 3) show a set of colorlight/searchlight
signals using double heads for limited speed and flashing yellow for
advance approach (1b and 1i), as well as using flashing for limited
speed and double yellow for advance approach (2b and 2i).
Furthermore, columns are shown for current Canadian practise (Ci) and
former Northern Pacific (Npi) practise, both of which have a number of
unique aspects for sequences of interlockings at various speeds.


Alternate colorlight head types:

                                -       -
       ---         ---        | R |   | R |
     / G Y \     / Y G \      | Y |   | Y |       -----       -----
     \  R  /     \  R  /      | G |   | G |     | G Y R |   | G Y R |
       ---         ---          -       -         -----       -----
        |           |           |       |           |           |
       ---         ---          -       -           |           |
     / G Y \     / Y G \      | Y |   | Y |       -----       -----
     \  R  /     \  R  /      | G |   | G |     | R Y G |   | G Y R |
       ---         ---        | R |   | R |       -----       -----
        |           |           -       -           |           |
       ---         ---          |       |           |           |
     / G Y \     / Y G \        |       -         ---           ---
     \  R  /     \  R  /        |     | R |     | R Y |       | Y R |
       ---         ---          |     | Y |       ---           ---
        |           |           |     | G |         |           |
        |           |           |       -           |           |
        |           |           |       |
        |           |           |       |         block    interlocking

           cluster            block interlocking

??  confirm which roads            C&O                   C&NW
    used each type


??  cluster head with lunar                                where does lunar
    4 light cluster head?                                  go for 4-light


Typical colorlight heads (same config for cluster & searchlight):

     -              -              -              -              -
   | G |          | G |          | G |          ( R )          | G |
   | Y |          | Y |          | Y |            -            | Y |
   | R |          | R |          | R |            |            | R |
     -              -              -              -            | U |
     |              |              |            | G |            -
     |           -  |              -            | Y |            |
     |         | G ||            | Y |          | R |            -
     |         | R ||            | R |            -            | G |
     |           -  |              -              |            | Y |
     |              |              |              |            | R |
     |              |              |              |            | U |
     |              |              |              |              -
     |              |              |              |              |
     |              |              |              |              -
     |              |              |              |            | G |
     |              |              |              |            | Y |
     |              |              |              |            | R |
     |              |              |              |            | U |
     |              |              |              |              -
     |              |              |              |              |
   block          block       interlocking   interlocking   interlocking
                 approach        begin        at end of        fully
                  siding         siding         siding         loaded


2.1.2 Position Light Signals

Position light signals were introduced by the Pennsylvania Railroad in
1915 to replace semaphore signals, consisting of rows of 4 lights
which directly mimiced 3 upper quadrant semaphore arms.  They were
modified to the present form in 1920, and consist of two large heads.
The top head is typically circular, 54" in diameter with a small pale
yellow light in the center and a maximum of 8 pale yellow lights in a
circle, centers 18" from the center of the head.  The aspect is
displayed by a line of 3 lights: vertical for clear, diagonal for
approach, and horizontal for stop.  the outer horizontal lights have
been replaced by red in some installations, particularly in lines west
of Pittsburgh.  The lower quadrant diagonal lights are normally blank
in the top head, and display the restricting indication in the lower
head.  The lower head displays the medium speed clear and slow speed
approach aspects.  The lower head never has the horizontal lights
fitted, and consists either of a bar supporting only the row of lights
needed, (or two bars for both diagonal lines), an oval head for a
single row of lights, or a circular head with the sides cut off.  If a
only a single light is needed for the stop and proceed indication, it
is mounted with a small circular head or none at all.  Limited speed
is indicated by a small fixed yellow reflective plate, which changes
the medium speed indication to limited, or by the flashing of the
lights in the lower head.

A smaller ground version of the signal consists of an array of white
(and red for stop in some installations) lights in a black rectangular
head.  The indications are given by pairs of lights giving indications
identical to (except for limited speed) the high signals.  Dwarf
signals consist of white lights providing slow speed indications, or
allowing the restricting indication depending on the head
configuration.

Amtrak has been slowly replacing the pale yellow lenses with color,
converting the signals to color position light.  In these signals
pairs of lamps in each haed show the aspect.

A variant of the PRR position light signals is used on some ex-N&W
(Norfolk and Western) lines.  Originally these consisted of rows of 3
yellow lights, as on the PRR, but later were modified to present 2
lights in a row, with the upper head containing colored lights.  The
bottom head (if present) is always a large circular target identical
to the top head, unless it only the center red light.


Typical position light heads:

    ---            ---            ---            ---            ---
 /   Y   \      /   Y   \      /   Y   \      /       \      /   Y   \
       Y              Y              Y                             Y
| Y  Y  Y |    | Y  Y  Y |    | Y  Y  Y |    | Y  Y  Y |    | Y  Y  Y |
   Y              Y              Y                             Y
 \   Y   /      \   Y   /      \   Y   /      \       /      \   Y   /
    ---            ---            ---            ---            ---
     |              |              |              |              |
     |              |              |              |              |
     |              -              |             ---            ---
     |            | Y |             /  \      /   Y   \      /   Y   \
     -            |   |           /  Y |      |     Y |      | Y   Y |
   ( Y )          | Y |         /  Y  /       |   Y   |      |   Y   |
     -            |   |        | Y  /         | Y     |      | Y   Y |
     |            | Y |        \  /           \   Y   /      \   Y   /
     |              -              |             ---            ---
     |              |              |              |              |
     |              |              |              |              |
     |              |              |              |              |
     |              |              |              |              |
     |              |              |              |              |
     |              |              |              |              |

    ---            ---            ---            ---            ---
 /   Y   \      /   Y   \      /   Y   \      /       \      /   Y   \
       Y              Y              Y                             Y
| R  Y  R |    | R  Y  R |    | R  Y  R |    | R     R |    | R  Y  R |
   Y              Y              Y                             Y
 \   Y   /      \   Y   /      \   Y   /      \       /      \   Y   /
    ---            ---            ---            ---            ---
     |              |              |              |              |
     |              |              |              |              |
     |              -              |             ---            ---
     |            | Y |             /  \      /   Y   \      /   Y   \
     -            |   |           /  Y |      |     Y |      | Y   Y |
   ( Y )          | Y |         /  Y  /       |   Y   |      |   Y   |
     -            |   |        | Y  /         | Y     |      | Y   Y |
     |            | Y |        \  /           \   Y   /      \   Y   /
     |              -              |             ---            ---
     |              |              |              |              |
     |              |              |              |              |
     |              |              |              |              |
     |              |              |              |              |
     |              |              |              |              |
     |              |              |              |              |

   block          block       interlocking   interlocking   interlocking
                 approach        begin        at end of        fully
                  siding         siding         siding         loaded



Norfolk and Western (N&W)  typical color position light heads

    ---            ---            ---            ---            ---
 /   G   \      /   G   \      /   G   \      /       \      /   G   \
       Y              Y              Y                             Y
| R     R |    | R     R |    | R     R |    | R     R |    | R     R |
   Y              Y              Y                             Y
 \   G   /      \   G   /      \   G   /      \       /      \   G   /
    ---            ---            ---            ---            ---
     |              |              |              |              |
     |              |              |              |              |
     |             ---            ---            ---            ---
     |          /   Y   \      /       \      /   Y   \      /   Y   \
     -                               Y              Y          Y   Y
   ( R )       |    R    |    |    R    |    |         |    |    R    |
     -                           Y              Y              Y   Y
     |          \   Y   /      \       /      \   Y   /      \   Y   /
     |             ---            ---            ---            ---
     |              |              |              |              |
     |              |              |              |              |
     |              |              |              |              |
     |              |              |              |              |
     |              |              |              |              |
     |              |              |              |              |

   block          block       interlocking   interlocking   interlocking
                 approach        begin        at end of        fully
                  siding/        siding/        siding         loaded
                crossover      crossover

Amtrak northeast corridor color conversions

    ---            ---            ---            ---            ---
 /   G   \      /   G   \      /   G   \      /       \      /   G   \
       Y              Y              Y                             Y
| R     R |    | R     R |    | R     R |    | R     R |    | R     R |
   Y              Y              Y                             Y
 \   G   /      \   G   /      \   G   /      \       /      \   G   /
    ---            ---            ---            ---            ---
     |              |              |              |              |
     |              |              |              |              |
     |              -             ---            ---            ---
     |            | G |        /       \      /   G   \      /   G   \
     -            |   |              Y        |     Y |      | U   Y |
   ( U )          | U |       |    U    |     |       |      |   U   | 
     -            |   |          Y            | Y     |      | Y   U |
     |            | G |        \       /      \   G   /      \   G   /
     |             ---            ---            ---            ---
     |              |              |              |              |
     |              |              |              |              |
     |              |              |              |              |
     |              |              |              |              |
     |              |              |              |              |
     |              |              |              |              |

   block          block       interlocking   interlocking   interlocking
                 approach        begin        at end of        fully
                  siding/        siding/        siding         loaded
                crossover      crossover


2.1.3  Color Position Light Signals

The positional aspect is displayed on a large circular head with pairs
of lights indicating the positional aspect: 2 horizontal red for stop,
2 diagonal yellow for approach, 2 vertical green for clear, and 2
diagonal (lower quadrant) lunar white for restricting.  White modifier
lights indicate the speed (above for full, below for medium) or speed
approached (above left for medium, below left for slow).  Yellow
modifier lights indicate concatenated interlockings (upper right for
medium approach medium and lower right for medium approach slow).  The
absence of any modifier indicates slow speed or an absolute stop.
Limited speed is indicated by a fixed yellow triangular marker which
changes the medium indication to limited, or by the flashing of the
medium speed white modifier.  Dwarf signals are identical in
configuration (but smaller) than the high signals.  Color position
light signals are used on the former B&O (Baltimore and Ohio) sections
of the CSX railroad, and on the SPCSL (Southern Pacific Chicago--St.
Louis line: formerly CMNW formerly ICG formerly GM&O formerly
Alton/B&O).
                                                             
Typical color position light heads:

     ^              ^              ^              ^              ^
     |              |              |              |              |
    (W)       (W)==(W)            (W)             |        (W)==(W)==(Y)
     |              |              |              |              |
     |              |              |              |              |
    ---            ---            ---            ---            ---
 /   G   \      /   G   \      /   G   \      /   G   \      /   G   \
       Y              Y              Y          U   Y          U   U
| R     R |    | R     R |    | R     R |    | R     R |    | R     R |
   Y              Y              Y              Y   U          Y   U
 \   G   /      \   G   /      \   G   /      \   G   /      \   G   /
    ---            ---            ---            ---            ---
     |              |              |              |              |
     |              |              |              |              |
     |              |             (W)            (W)       (W)==(W)==(Y)
     |              |              |              |              |
     |              |              |              |              |
     |              |              |              |              |
     |              |              |              |              |
     |              |              |              |              |
     |              |              |              |              |
   block          block       interlocking   interlocking   interlocking
                 approach        begin        at end of        fully
                 siding/         siding/        siding         loaded
                crossover      crossover


2.2  SIGNAL SYSTEMS AND LOCATION

North American operations are typically right main for double track
running.  (A notable exception is the C&NW which runs left main.)
Signals are located immediately to the right or above the track
signaled.  If there are intervening tracks between the signal mast and
the signaled track, dummy masts (dolls) are used which contain a blue
light or reflector.


Block signal          |---O
                      |---O     approach interlocking
                         O

Interlocking signals  |---OO    full and medium speed
                      |--O-O    full and slow speed
                      |--OOO    full, medium, and slow speeds

                                *  red + yellow + green
                                X  red + yellow
                                +  red + green
                                |  red



2.2.1  Manual block

Operators at widely spaced block stations (5--15 mi) control signals
which allow entry to blocks.  Controlled manual block, with track circuits
and interlocks between adjacent block stations, was never common in North
America.  Manual block signaling has been (totally?) replaced by other means.


2.2.2  ABS -- Automatic Block Signals

Block signals automatically controlled by track circuits.
Interlockings are still controlled by operators at or near each
interlocking.  3 block-aspect signaling is by far the most common.

Typical block signal masts:

       ^                   ---            ---              ^
       |        -       /   Y   \      /   G   \           |
       -      | G |           Y              Y             |
     ( S )    | Y |    | Y  Y  Y |    | R     R |         (W)
       -      | R |       Y              Y                 |
       |        -       \   Y   /      \   G   /           |
       |        |          ---            ---             ---
     - |      - |           |              |           /   G   \
   ( S )    | Y |           |              |                 Y
     - |    | R |           |              |          | R     R |
       |      - |           |              |             Y
       |        |           |              |           \   G   /
       |        |          (Y)            (R)             ---
       |        |           |              |               |
       |        |           |              |               |
       |        |           |              |               |
       |        |           |              |               |
       |        |           |              |               |
       |        |           |              |               |
       |        |           |              |               |
       |        |           |              |               |
       |        |           |              |               |

    search-   color      position       position         color
     light    light       light          light          position
                          (PRR)          (N&W)           (B&O)

    Note: lower colorlight or searchlight head only used if necessary for
    4 block-aspect signaling or in approach of interlockings; searchlight
    colors match colorlight colors



  Block and signal locations:

  blocks are typically 1 -- 3 mi long
  signals default to clear


         O--|             O--|             O--|             O--|
  <------------------------------------------------------------------
  ------------------------------------------------------------------>
            |--O             |--O             |--O             |--O

            |     block      |    block       |     block



  2 position signaling (semaphore)

  indication                 clear       approach         stop
                                                      (permissive)
  ------------------------------------------------------------train->
  aspect        g|            g|            g|            r--
                gV            gV            y-<           y-<


  3 position signaling (semaphore)

  indication                 clear       approach         stop
                                                      (permissive)
  ------------------------------------------------------------train->
  aspect        g|            g|            y/            r--


  3 position signaling (colorlight/searchlight)

  indication                 clear       approach         stop
                                                      (permissive)
  ------------------------------------------------------------train->
  aspect         G             G             Y             R



  3 position signaling (position light)

  indication                 clear       approach         stop
                                                      (permissive)
  ------------------------------------------------------------train->
  aspect         Y             Y              Y
                 Y             Y             Y            YYY
                 Y             Y            Y
                                                           Y



  3 position signaling (color position light)

  indication                 clear       approach         stop
                                                      (permissive)
  ------------------------------------------------------------train->
  aspect         W             W             W             W
                 |             |             |             |
                 G             G              Y           R R
                 G             G            Y



  4 position signaling (colorlight/searchlight using flashing yellow)
     high speed / high density

  indication   clear        advance      approach         stop
                            approach                  (permissive)
  ------------------------------------------------------------train->
  aspect         G             Y*            Y             R



  4 position signaling (2 head colorlight/searchlight using double yellow)
     high speed / high density

  indication   clear        advance      approach         stop
                            approach                  (permissive)
  ------------------------------------------------------------train->
  aspect         G             Y             Y             R
                G             Y             R             R





2.2.3 Interlocking Signals

Interlocking signals are controlled by operators in towers typically located
at or near the interlocking.
Fully loaded interlocking signal masts:

       ^       ---         ---            ---              ^
       |      | G |     /   Y   \      /   G   \           |
       -      | Y |           Y              Y             |
     ( S )    | R |    | Y  Y  Y |    | R     R |    (W)==(W)==(Y)
       -      | U |       Y              Y                 |
       |       ---      \   Y   /      \   G   /           |
       |        |          ---            ---             ---
       -       ---          |              |           /   G   \
     ( S )    | G |         |              |             U   Y
       -      | Y |        ---            ---         | R     R |
       |      | R |     /   Y   \      /   Y   \         Y   U
       |      | U |     | Y   Y |        Y   Y         \   G   /
       -       ---      |   Y   |     |    R    |         ---
     ( S )      |       | Y   Y |        Y   Y             |
       -       ---      \   Y   /      \   Y   /           |
       |      | G |        ---            ---        (W)==(W)==(Y)
       |      | Y |         |              |               |
       |      | R |         |              |               |
       |      | U |         |              |               |
       |       ---          |              |               |
       |        |           |              |               |
       |        |           |              |               |

    search-   color      position       position         color
     light    light       light          light          position
                          (PRR)          (N&W)           (B&O)



  Typical signal locations for a passing siding (signals for only one 
     direction shown):


   -------------------*--------------------------------------*------
    |---*      |---X*  \                             |---|* /
       +                ------------------------------------
                                                     |---*|

   (color position light)

   ------.------------*--------------------------------------*------
    |---*.     |--.*.  \                             |---*. /
                        ------------------------------------
                                                     |--.*



  Signaling for straight (normal speed) route (typical colorlight/searchlight):

                                                       G
                                                       R
                                                    |
                                                    |  clear
                                                    /
    O--|              O--|                         /   OO--|
  <-----------------------------------------------X------------------

  ----------------------------------------------*------------------->
       |--O              |---O            |--OO
                            O

       clear             clear            clear

         G                  G               G
                           G                R


  Signaling for medium speed diverging route (typical colorlight/searchlight):

                                                         R
                                                         R
                                                    |
                                                    |  absolute stop
                                                    /
    O--|              O--|                         /   OO--|
  <---------------------------------------------- X -----------------
                                                 /
  ----------------------------------------------* ------------------>
       |--O              |---O            |--OO
                            O

       clear             approach         medium
                         medium           clear

         G                  Y               R
                           G                G


  Signaling for slow speed diverging route (typical colorlight/searchlight):

                                                         R
                                                         R
                                                    |
                                                    |  absolute stop
                                                    /
    O--|              O--|                         /   OO--|
  <---------------------------------------------- X -----------------
                                                 /
  ----------------------------------------------* ------------------>
       |--O              |---O            |--OO
                           O

       clear             approach         slow
                         slow             clear

         G                  Y               R
                                            R
                           G                G



  Signaling for straight (normal speed) route (typical position light):

                                                         Y
                                                         Y
                                                         Y
                                                    |
                                                    |  clear
                                                    /
    O--|              O--|                         /   OO--|
  <-----------------------------------------------X------------------

  ----------------------------------------------*------------------->
       |--O              |--=O            |--OO

       clear             clear            clear

         Y                 Y                Y
         Y                 Y                Y
         Y                 Y                Y


  Signaling for medium speed diverging route (typical position light):

                                                        YYY
                                                    |
                                                    |  absolute stop
                                                    /
    O--|              O--|                         /   OO--|
  <---------------------------------------------- X -----------------
                                                 /
  ----------------------------------------------* ------------------>
       |--O              |--=O            |--OO

       clear             approach         medium
                         medium           clear

         Y                  Y
         Y                 Y               YYY
         Y                Y
                           |                |
                           Y                Y
                           Y                Y
                           Y                Y


  Signaling for straight (normal speed) route (typical color position light):

                                                       W
                                                       |
                                                       G
                                                       G
                                                    |
                                                    |  clear
                                                    /
    O--|              O--|                         /    O--|
  <-----------------------------------------------X------------------

  ----------------------------------------------*------------------->
       |--O              |--O             |--O

       clear             clear            clear

          W                 W                 W
          |                 |                 |
          G                 G                 G
          G                 G                 G


  Signaling for medium speed diverging route (typical color position light):

                                                      R R
                                                    |
                                                    |  absolute stop
                                                    /
    O--|              O--|                         /    O--|
  <---------------------------------------------- X -----------------
                                                 /
  ----------------------------------------------* ------------------>
       |--O              |--O             |--O

       clear             approach         medium
                         medium           clear

          W               W-
          |                 |                 |
          G                 G                 G
          G                 G                 G
                                              |
                                              W


  Signaling for slow speed diverging route (typical color position light):

                                                      R R
                                                    |
                                                    |  absolute stop
                                                    /
    O--|              O--|                         /    O--|
  <---------------------------------------------- X -----------------
                                                 /
  ----------------------------------------------* ------------------>
       |--O              |--O             |--O

       clear             approach         slow
                         slow             clear

          W                  -Y
          |                 |                 |
          G                 G                 G
          G                 G                 G


2.2.4  APB -- Automatic Permissive Block signaling

Track circuits detect direction of movement, and provide signaling for
single track between passing sidings.  All signals opposing movement
on a stretch of single track are set to stop, including the absolute
stop signal at the entrance to the single track from the double track
passing siding.  This provides adequate protection for opposing
movements without excessive block overlap for following movements.





AUTHOR INFORMATION

Please send corrections and comments to:

James Sterbenz
jpgs@watson.vnet.ibm.com
IBM HPCC MS93
472 Wheelers Farms Rd.
Milford CT 06460-1844
USA
+1 203 783 4380

382 Denslow Hill Rd.
Hamden CT 06514-2006
USA
+1 203 783 4380


CREDITS

The following provided helpful comments and sources of information.

Martin E. Pope
Jon Roma
Dave Pierson



REFERENCES (incomplete list so far!)

Elements of Railway Signaling, General Railway Signal / General Signal Corp.,
Rochester, New York, 1979.

Armstrong, John, "All About Signals", Trains, June and July 1957, Kalmbach
Publishing Co., reprinted by Kalmbach Books.

Wesscott, Linn H., "Signal Aspects and Indications", Trains, Vol.57 #10,
Oct. 1979, Kalmbach, Milwaukee Wisconsin, pp. 54--57.

Signals and Signal Symbols, Train Shed Cyclopedia #27, Newton Gregg, Novato
California, reprint from The 1911 Railway Signal Dictionary, 2nd ed., B.B.
Adams and Rodney Hitt eds., Railway Age Gazette.


Chessie System Operating Rules (The Chesapeake and Ohio Railway
Company and the Baltimore and Ohio Railroad Company. April 27, 1969

Chicago and North Western System (Chicago and North Western Railway
Company; Chicago, St. Paul, Minneapolis and Omaha Railway Company),
Rules of the Operating Department, 1 January 1953

Conrail Rules for Conducting Transportation, 28 April 1968 (rev.
1 July 1975)

The Consolidated Code of Operating Rules (adopted by: BN, ... MILW
SOO, WSOR, LS&I), 1980.
General Code of Operating Rules (Adopted by: UP, MP, MKT, SSW, SP,
ATSF), 28 April 1985.

Norfolk Southern Corporation Operating Rules, 1 Feb. 1985.

United States War Department, U.S. Army Corps of Engineers, 
Military Railways, Document #539, Professional Papers #32, U.S Government
Printing Office, Washington, revised edition, 1917.

Pennsylvania Railroad System Operating Department Rules for
Conducting Transportation, 26 April 1925

Southern Pacific Company (Pacific Lines) Rules and Regulations of
the Transportation Department, 1 July 1960.

Southern Pacific Company (Pacific Lines) Rules and Regulations of
the Transportation Department, 31 Oct. 1976 (rev. 26 Oct 1980)

--------
Copyright 1992
Please DO NO COPY OR REDISTRIBUTE this article...
A more complete version will come soon.


--
James Sterbenz  IBM HPCC       (High Performance Computing and Communications)
                472 Wheelers Farms Rd. MS93           jpgs@watson.vnet.ibm.com
                Milford CT 06460-1844 USA                      +1 203 783 4380

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