Articles from RPO V3 N4
Dispatcher’s Desk
by Bob Amsler
One item in railroading that is not often in the
magazines is a discussion of signals and how they work
on the prototype. What is often said by the modeler is
"I operate in the dark," or with CTC, or ABS,
without much discussion of what these are and what they
do. Perhaps it is best to make the analogy that these
are the traffic signals that railroads use. However, it
is not a perfect analogy.
Signals are the tools used by dispatchers to manage
the flow of traffic over a rail line. Dispatchers can
dispatch by train orders or with centralized traffic
control (CTC). Train orders are used in dark
territories, or rail lines without any signals.
Trains travel from one station or passing siding to the
next based on orders given by the dispatcher through
station agents or others. With signals the trains do not
need to use train orders to travel from one place to the
other, because the signal will tell the engineer what he
needs to do. Centralized traffic control (CTC)
allows the dispatcher to let the engineer run the train
without stopping for orders. It allows for quicker and
more efficient running of trains by controlling
interlockings. Automatic block signaling (ABS)
does the same thing for the traffic flow but with block
signals.
There are two types of signals: block and
interlocking. A block signal is a signal which
controls a block of territory on a rail line. The signal
will have only one head or light on it. Its job is to
control the distance between points. Therefore, the
color of the signal governs the speed of the train.
Green will allow the train to travel to either its
maximum speed or the maximum speed of the line,
whichever is less. It also indicates that the next block
is clear. Yellow is a restrictive indication which means
that the train must travel at a medium speed and that
the next block is occupied. Therefore, the train must be
prepared to stop at the next signal. Remember that
blocks are as long as the operations department thinks
it will take a train to stop within it.
Red means that the train must stop -- sometimes. This
is because there are two types of block signals:
absolute and permissive. An absolute signal means
that the train must stop and not proceed past the signal
(absent special limited circumstances) until the aspect
changes. A permissive signal means that the train
must make a stop but can then proceed into the next
block at a restricted speed, always prepared to stop
within half the engineer's sight distance. One special
type of permissive signal is a signal which is on a
grade. This permissive signal allows a heavy, slow train
to proceed past the signal without having to stop and
restart the train. A permissive block signal will have a
number plate or a plate with the letter "P" or
"G" (for grade) on it.
Automatic block signals are block signals which
are joined by electrical circuits to provide a signal
indication. A block signal at A will display a red,
yellow, or green signal depending on whether the next
block signal at B is showing red, yellow or green. If a
train is between signal A and signal B, then the signal
at A will show red. If a train is between signal B and
signal C, then B will indicate red and A will indicate
yellow. If a train is between signal C and signal D,
then C will show red, B will show yellow and C will
indicate green. This insures that the trains are
appropriately spaced for safety.
An interlocking signal is a signal which
governs an interlocking plant. An interlocking plant is
the joining of two or more tracks. Signals exist at
these points to control movements through these plants
and to provide for safety for the trains and their
crews. These are the signals that a dispatcher controls
with CTC. They are marked by having multiple heads or
signal lights. A red signal at an interlocking plant is
always considered to be absolute. A train cannot move
through an interlocking signal which is red. The top
signal head indicates whether a train will be allowed to
proceed through the interlocking on the main and, if so,
at what speed. The second signal indicates whether the
train will be allowed to move through the interlocking
on the diverging route and, if so, at what speed. The
bottom head on a triple headed signal protects movements
though inferior diverging routes.
The top signal means the same as a block signal. Red
means stop. Yellow means proceed at medium speed
prepared to stop at the next signal. Green means go. If
the top signal is red, the maximum speed for moving
through a diverging route is medium. If the second head
is green, this is a medium clear and the train can move
through the interlocking at medium speed and the next
signal is either clear (green) or approach (yellow). If
it is yellow, (medium approach) the train can move
through the diverging route at a medium speed but must
be prepared to stop at the next signal, and if it is
red, the train must stop. If the top and second heads
are red and the third head is green, this is a slow
clear, which means that the train can proceed through
the interlocking at a slow speed on an inferior
diverging route. If the signal is yellow, a restricting
signal or a call on indication, the train can proceed on
the inferior diverging route at a restricted speed
prepared to stop. Red again means the train must stop.
A call on indication allows a train to enter an
occupied plant to make switching moves. It is also used
to move trains through an interlocking when the
mechanisms governing the interlocking are broken, to
allow trains into unsignaled track, to allow trains to
move from unsignaled track onto the interlocking plant
and to allow a train to enter one-way track against the
flow of traffic.
The prototype will also use dummy heads on its
signals to indicate that the signal is an interlocking
signal. The dummy head is always red. The dummy head is
the signal head for the track that it is physically
impossible for a train to take. For instance, when two
tracks meet, one a main line, the other a secondary
main, trains approaching from two of the three
directions cannot physically take another path. A train
approaching the switch can continue on the main or
diverge to the secondary. This will need to be a two
headed signal with all signals working.
The train on the main approaching from the other
direction cannot take the diverging route. Therefore,
the bottom signal is unnecessary and a dummy head can be
used to note that it is an interlocking signal. If the
train is coming from the secondary line to the main, it
can only go in one direction; therefore, the top head is
a dummy.
The dispatcher controls the interlocking plants and
the ABS functions in between these two areas. The
dispatcher then has a control panel with the
interlockings marked on it, but not the block signals
because he does not control these. In addition, the
distances between the interlockings are not noted by
similar distances on the control panel. The distances
between the two areas are more or less uniform.
Therefore the dispatcher must be aware of the distances
between the interlockings.
This is the way that the prototype signals its lines.
It can be modeled on our own pikes and incorporated in
the way we run operations. Unfortunately, I cannot
provide the information about building the circuits
because I do not understand electronics. I need someone
to explain electronic things to me -- real slowly. But
please look at Richard Schumacher's articles to get
started.
Until the next time, I hope all the signals you see
are green over red.
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