North American Signaling:
Automatic Block Signaling, ATSF "Flat Pair" ABS
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A number of railroads, including the Atchison, Topeka & Santa Fe
Railway, implemented a special
variety of single track Automatic Block Signaling, that the ATSF
refered to as "Flat Pair" signaling. "Flat Pair" refers to the
intermediate block signals standing back-to-back in pairs, as opposed
to the "Staggered Pairs" on the older Opposing Overlap system (I'll get
around to describing that eventually...). The "Flat Pair" ABS is
technically in essence an Absolute
Permissive
Block (APB) signaling , but
consistent with practices on (some) western roads it had all
permissive signals. In later years, however, the number plates have
been removed from the headblock signals, thus making them absolute
signals as on an ordinary APB.
What technically differentiates the "Flat Pair" ABS from ordinary
APB is the arrangement of signals around sidings. "Flat Pair" ABS
features an arrangement of signals around sidings, with automatic
functions for meets and passes, that are unique from ordinary APB. The
arrangement is also found on the ATSF version of the older Opposing
Overlap ABS (which I will some day get around to explain in another
document...). I have seen similar systems in operation on lines
formerly by the Texas & Pacific and the Southern Pacific.
Before reading further, it is recommended to become familiar with
the method of operating trains on single track, the "tumble-down" and
the protection of opposing trains around sidings on the standard Absolute Permissive
Block (APB) signaling .
The arrangement of signals around a siding in ATSF Flat Pair
territory is as shown below:
![](t4_norm.gif)
The siding switches are spring switches,
their normal position being
supervised
in the main track signals. The siding is not equipped with track
circuits,
thus the ABS does not "know" if there is a train in the siding or not.
As mentioned above, the signals on the main track are all
permissive, i.e. their most restrictive
indication
is "Stop and Proceed". A train with the proper authorization may
proceed
past such a signal at red, at Restricted Speed. The signals in the
siding
are, however, absolute signals, i.e. red means "Stop". These
signals may not be
passed
when red, except after permission from the dispatcher or after other
precautions.
All signals are green as their normal aspect, i.e. when no train is on
main track and switches are normal (the spring switches allow trains to
move from siding as well as main track).
In order to illustrate the features of ATSF Flat Pair ABS, let's follow
a train across a section of a Flat Pair line:
![](atsf_flatpair_norm.gif)
To the left is one end of a siding, then
a section of intermediate block signals, then another siding and a
piece of the line towards the next siding to the right. As with most
ABS most signals are green when the block is at rest (i.e. no trains
nearby).
First, a train is approaching from the left. For now it has not
progressed past the siding switch at the left siding, so no tumble-down
has taken place yet:
As the "Headblock Signal" is located before the siding switch,
occupying the switch occupies the first block, and thus triggers
the tumble-down:
![](atsf_flatpair_02.gif)
Please note what happened at the next
siding: Not only did the Headblock signal drop to red, but so did
both signals in the siding (the Leave
Siding signals). The leftmost Leave Siding signal is of course a
mirror of the Headblock signal on the main track (as they protect
against the same train movement from the left), but the rightmost Leave
Siding signal deserves an extra bit of expanation.
In the situation above, the rightmost Leave Siding signal establishes
protection against a train that has gone in the siding to be overtaken.
Remember that in Automatic Block Signaling territory, trains in the
same direction are to be protected by the
signals alone, and this is the reason why the signals in the siding
needs to be absolute signals. By dropping the signal to red, our train
from the left is protected against any train leaving the siding. And by
establishing the protection so early there is enough time that a train
in the siding can accept the Leave Siding signal just as it drops,
without endangering our train train coming from the left on the main
track. This principle is
illustrated in my document on
ATSF
double track ABS .
As the train moves across the line, the signals clear behind it (as
on ordinary APB). Note that the Headblock signal and the Leave Siding
signal at the leftmost siding share the same aspect:
![](atsf_flatpair_03.gif)
There are several different ways to
build the circuits that detect the direction of the train. These
circuits decide when to tumble signals down and when to enable the
signal in the rear to clear up after the train. One of these varieties
of circuits
keep the opposing intermediate block signal behind the train at red,
until the train has cleared the block. This seems to be the prefered
implementation
that the ATSF used. The choice of circuits seems to be a matter of
preferences at the railroad or the manufacturer, but keeping the rear
opposing signal at red does prevent a reversing train from picking up
much
speed before it has triggered the tumble-down in the opposite direction.
The train continues, now approaching the overlap section for the
Entering signal by the siding:
Then entering the overlap:
And finally the train clears the "entry" switch...
![](atsf_flatpair_07.gif)
...and then clears the Leave Siding signal behind it to let out any
meeting train in the siding. The signal on the main track stays at red,
thus protecting a train leaving the siding:
![](atsf_flatpair_11.gif)
Next interesting change is when signals
around the leftmost siding
switch
clear. When the signal at the point of the siding switch goes back to
green,
the opposite signal on the main is released and changes to the same
aspect
as the siding signal, in this case green. At the same time, the train
is far anough away that the signals at the right end of
the siding changes to yellow. Since there's no train behind them, thus
at most a train in the siding, but simply both change to yellow:
![](atsf_flatpair_12.gif)
And everything back to normal:
And now to the siding switches, and their influence on the signals:
In general, ABS signals supervise the switches. An ABS signal
governing
movement over a switch in an unsafe position (i.e. not lined to the
Main Track), must show its most
restrictive
aspect. In practical terms, a check for a switch' normal position is
included
with a check it the track circuit, so opening a switch is that same as
occupying the track section that the switch is in:
Reversing the switch is in principle the same as opening it. It is,
however,
possible to gain a little operational advantage by checking for the
switch
being reversed. A reversed switch is something that happens if a train
is lining itself into the siding:
By utilizing the knowledge that the train is heading for the siding,
the
ABS can disregard the occupied overlap for the Entering signal in the
other end of the siding, and allow this train to clear. Thereby letting
the meeting train approach at a higher speed than "Stop and Proceed"
would
allow.
Without trains to blur the effect of the open switch:
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Text, Images, HTML: Carsten S. Lundsten.