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W&H MAIN YARDS: Railroad Signal FAQ

By Dave Pierson
Last Updated 3-22-95

    V0.4  hacked in positionlite

    V0.3 Modified to include searchlight/colorlight aspects used by NORAC.
    Does not include Position_Light, Modified_PL (I), or modified_PL (II),
    or Color_Position_light.  Quasi sorted by rule number.

    V0.2.  Removed the {tabs} that were confusing some systems.
    Incorporated comments received.

    Some indications and meanings.  Derived from PennCentral practice, there
    is some, tho NOT complete commonality across RRs.  Transit systems
    tend to be even more individualistic.  This is DRASTICALLY simplified, to
    keep the length managable.  US Interlocking Signalling is generally
    described as "speed signalling" in that each aspect conveys an unambiguous
    speed at which the trains should operate.  This interacts with the "block
    signalling, to display "zero" (STOP_AND_STAY) when the block signalling
    requires it.  Speeds FULL, LIMITED, MEDIUM, SLOW are defined in the
    Employee TT, for each stretch of track and for each class of train.

    The indication may involve one, two or three heads, either "high"
    or "dwarf" (lo mounted, trackside).  While indications are similar
    and related, they need not be identical.  BN means aspect used by BN.
    CCOR means aspect used by Consolidated Code of Operating Rules, which
    governs many Western US railroads.  The BN or CCOR Indication/rule may
    vary slightly, but i feel the "sense" is the same.  N is NORAC rules,
    a common set covering Conrail, AMTRAK, etc.  Trying to keep this
    Rule numbers are from NORAC, 1991.

      C Three Head High Signal       C Two Head High Signal.
      C                              C
      C   C Two Head "dwarf" Signal  |   (etc....)
      |   C (Same indication/rule)   |

    See bottom for position lite discussion.

N   BN  CCOR   PC Aspect        Name                    Indication

X   X     X    :  G G              CLEAR              Proceed at maximum
               |  R R      :                          authorized speed (281)
               |  R | G   (d)
               |  | | G G  |

X   X     X    -  R R              MEDIUM CLEAR       Proceed at medium speed
               :  G G      -                           (usually, half maximum)
               |  R | G    :                           (283)
               |  | | R(f) |

X   X     X    /  Y Y              APPROACH           Approach next signal
               |  R R                                 prepared to stop. (285)
               |  R |
               |  | |

               \  Y  
              (m) YY \            CAUTION             Train Exceding Medium
               |     (m)                              Speed must reduce to that
               |     |                                speed.  Approach next
                                                      signal prepared to stop.
                                                      Approach next facing point
                                                      switch prepared to stop.

X   X     X    _  R R              MEDIUM APPROACH    Proceed at MEDIUM speed
               /f Y Y                                 prepared to stop at
               |  R | Y                               next Signal. (286)
               |  | | R(f)

X                 R R              SLOW CLEAR         Proceed, not exceeding
                  R G                                 SLOW speed, usually 15
                  G | G                               MPH. (287)            
                  | | R :

X               - R   R            RESTRICTING        Proceed not exceeding
                \ R   Y -                             SLOW speed, expecting to
                | Y R | \                             find track occupied,
                | | Y | | \                            switch thrown against
                                                      you, etc. (290)

X   X     X    - R R           STOP_AND_PROCEED       The "#" indicates the
              (m)R R                                  presence of a "number
               | R | #   -                            plate", which makes it
               | # # R # (m)                          Stop and Proceed,
               | | | R R |                             expecting to find track
                                                      occupied.  (291)

X   X    X     - R R            STOP_SIGNAL           Stop_and_Stay (292)
               | R R     -
               | R | R  (d)
               | | | R R | -

    So far, so good.  The indications are fairly intuitive:  High/Medium/Low   
    speeds map to head positions.  These (mostly) govern thru interlockings,
    where allowable speed varies depending on the design of the switch, which
    path, etc.

    And this was the original (ca 1900 situation).  But there arose a
    need for more indications....

N   BN    CCOR    PC Aspect            Name                Indication

Y(f)               Y Y             ADVANCE APPROACH    Proceed expecting to
R                  Y Y                                 find next signal at
R                  R |                                 APPROACH. (281D)
|                  | |

X                / Y   Y           APPROACH MEDIUM     APPROACH next signal
                 : G Y G /                             at MEDIUM Speed. (282)
           X     | R G | :
                 | | | | |

                   R              MEDIUM APPROACH SLOW  MEDIUM speed, APPROACH
X*                 Y                                    next signal at SLOW 
                   G                                    speed.
                   |              NORAC: MEDIUM         MEDIUM speed APPROACH 
                                  APPROACH MEDIUM (!)   next signal at MEDIUM
                                                        speed. (283A)

n/a                R               MEDIUM ADVANCE      Proceed at MEDIUM Speed
                   Y                APPROACH           next signal at APPROACH.
                   Y                                   (Following signal at
                   |                                   STOP.  I find the rule
                                                       explanations ambiguous.)

n/a                G   G 
                   Y   Y           ADVANCE APPROACH    Proceed, Approach next
                   R G |           MEDIUM              Signal at MEDIUM Speed.
                   | Y |
    (note how arcane these are getting...)

Y Y              / Y               APPROACH SLOW        APPROACH next signal
Y R        X     / R /                                  at SLOW Speed. (284)
R G              | G /
| |              | | |

               _ R
               / R    -              SLOW APPROACH       SLOW speed, APPROACH
X              | Yf Y /                                  next signal prepared
               | |  R | /                                stop. (288)

       These change, over time, and from RR to RR.  If researching a particular
       RR, try to get their Book of Rules of the Operating Department and an
       Employee Time Table, which will discuss their practice.  At any one time,
       the indications in use on one RR are unambiguous.  Mostly.

       I have omitted the "flashing" aspects (mostly) , which move the speed
       allowed up one "increment": thus "medium clear" would become "limited

Position lights:
Not all the aspects of color signals were replicated.  As used, all are "two
head", tho orginally some 3 head forms were used.  In real life, a 5' or so
diameter circle of 8 pale yellow (for high) lights, with one more in center,
with black metal background.  Lower element may be skeletal, lamp units only. 
derived by mapping 3 aspect semaphore indications, evolved over the years.  Mid
50s "lines west" got two reds in the horizontal postion, with realy toi "kill"
the center lamp.  AMTRAK has been modifying the NEC to have two greens for
the vertical, two yellows for the diagonals & two reds for the horizontal.

This is not (quite) the B&O Color Position light scheme...

Pedestal (mid hight) and dwarf (groundmount) exist as note.

Odd Indications:
	8 bulb circle:
	variable phase break active.  Drop pans (ore reduce power).
	5 bulb X:
	take siding.  (may be obsolete...)

In the table:
		: == two vertical lights, green (three, yellow, pre AMTRAK)
		/ and \ two/three diagonal (actually 45 degree)
		- == two or three horizontal.
(There are older forms with 4 lites, dunno if any left...)
(f) means flashing.
(d) means (intentionally) dark
(m) means a single marker lamp.

"memory aids"

       "If its not all Red, its not Red at all".  That is, unless all
       heads are red, the reds become "place holders".  Concentrate on
       the other colors and their positions.

       The PC "course" most of this is lifted from, in a couple places,
       just gives up and says "this violates the memory rules, just
       remember it."  In the face of such expert advice, who am i to

       I have omitted much related info, to keep this of managable length.

A few (more) notes on switches.  Signals are located in advance of the switch
which they "protect", to allow the crew time to get the train to the right
speed.  Trains do not respond rapidly to control inputs.  Thus, signal location
is determined by authorized speed and grade, among other things. 

Switches can affect signals indications in several ways:

  If the switch is open, a train from a direction which is at risk must be
  signalled to stop.

  If a switch is set to allow an opposing train to make a conflicting move,
  other trains must be signalled to stop.

  If a switch is designed for high speed moves (longer radius of points, etc,
  the signal should give a less restricting indication.)

Thus, the same train, at the same point, may receive any of several different
indications, depending on conditions ahead.  Similarly, the same track layout
may be differently signalled, depending on switch design.

Some of the signals will be under remote control from an interlocking "tower"
or CTC (Centralized Traffic Control) board.  The operator typically selects
"clear" or "stop".  The aspect presented in the field will be determined by
local conditions: switch design, location of trains, setting of switches, etc. 
dave pierson			|the facts, as accurately as i can manage,
Digital Equipment Corporation	|the opinions, my own.
200 Forest St			|I am the NRA.
Marlboro, Mass 01751		|pierson@msd26.enet.dec.com
"He has read everything, and, to his credit, written nothing."  A J Raffles

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