CPL signals and how they operate

NW Mailing List nw-mailing-list at nwhs.org
Thu Jan 23 11:36:55 EST 2020

I found some of the information I have been looking for in a February 1940
article in Railway Signaling magazine (attached).  This article describes
the upgrade of the automatic block signaling equipment from semaphore to
position light with coded track circuits, from Norfolk to Petersburg.  Some
of the salient points for me were that the coding system used 110 V, 60 Hz
AC applied to track sections to convey occupancy information.  The AC
signal was interrupted 75 times per minute to indicate occupancy of the
next block in advance (Approach), 180 times per minute to indicate neither
the current nor the next block in advance were occupied (Clear) and, where
appropriate 120 times per minute to indicate a turnout in the next block in
advance was set for the diverging route (Approach Diverging).  Of course,
as failsafe, no signal or a constant voltage caused the signal to display
the Stop aspect.

What I am still wondering at this point is how widely this method of signal
communication/control was implemented over the N&W and how long it was in
operation before the next evolutionary step.  Did it make to the time that
CPLs supplanted PLs?

Jim Cochran

On Wed, Jan 15, 2020 at 6:42 AM NW Mailing List <nw-mailing-list at nwhs.org>

> Excellency:
> Notice that the circuits on the plans you provided are steady energy (i.e.
> non-coded) track circuits fed by rectified AC.  My guess is that these
> circuits were quite close to those used on the N&W.
> 1.)  What you see between Signals 425 an 403 are two "cut sections"
> between the block signals.  Cut Sections are provided when it is
> impossible, for whatever reason, to drive through a long block a current
> adequate to hold up the track relay at the far end of the block.  So the
> block is divided into several smaller sections by the insertion of
> additional insulated joints and battery feeds... "cut sections" as they are
> called. The drawback is that you need to provide energy (battery) at each
> cut section, but if your railroad has AC hung on its pole line, your
> problem is solved.  The length of the block could be one factor occasioning
> the need for cut sections.  Bad ballast leakage could be another e.g. a
> source of acid coming from a nearby industrial plant or acid rain causing
> rail-to-ballast leakage.  Or the cause could just be soggy ballast, as in a
> tunnel.  In the early days of automatic block signaling, the one-mile-long
> block was standard.  Those were lengthened over the years so that by the
> 1980s most blocks were two miles long.  I recall the signalmen saying that
> in a few places the track circuits had to be less than a thousand feet long
> because of ballast (leakage) conditions.  The sulphuric acid coming out of
> the stack of engines did not help out track circuits at all. And if the
> railroad hauled a lot of salt or iron ore pellets, keeping the track
> circuits adjusted was a constant job.  I do not think these conditions
> exist with today's electronic track circuits, driven by RF's (radio
> frequencies,) although I have virtually no experience with electronics
> track circuits or Electrocode.  I recall one experimental case where they
> were trying to drive an RF (radio frequency) track circuit through a 4 mile
> long test block on my Division back in the 1990s.  (Of course, a four mile
> long block is a shipwreck and foolishness for the operating guys.)
> Notice that the rail polarity is staggered (opposite) for each track
> circuit.  That is the fail safe protection in case of the breakdown of the
> insulation at an insulated joint, so that an energized track circuit does
> not falsely energize and pick up the relay on an adjacent track circuit
> that is supposed to be de-energized.
> 2.)  As to your questions about nomenclature...  On those sheets, M is
> nomenclature for a repeater.  SA is, I think, Slow Acting, indicating the
> type of relay.  As I recall, slow acting relays were used to **bridge** a
> momentary loss of shunt on the track circuit.  **Slow** was something like
> a half-second between de-energization of the coil and drop away of the
> armature..
> 3.)  Yes, the signals are approach-lit in the scheme given on these
> plans.  I don't think I saw any approach lit signals on the N&W in the old
> days.  But then guys on trains only encountered lit up signals, so how
> would they know?  Approach lighting was developed back in the days when
> everything in the field was done with primary batteries, in which case it
> saved a lot of money.
> 4.)  Notice that these plans are for single track ABP (Absolute Permissive
> Block.)  The N&W's Punkin Vine was wired as APB before they installed CTC
> in the late 1970s, although the N&W never used that term in its Special
> Instructions, Rule Book or other literature.  N&W no doubt had other APB
> single main tracks before CTC and Rule 261 came around, like the Bristol
> Line and the Clinch Valley.  I am just not sure of all the places.
> APB was a fascinatingly simple circuitry designed and parented by a US&S
> Signal Engineer named Sedgwick North Wight (sic.)   I think he was called
> "the Little Guy with the Short Pencils."  His patent was filed in 1916 and
> issued in 1919, but I have read that he worked out the idea for APB in
> 1914.  The essence of the APB circuitry is that a train entering at one of
> a segment of single track knocks down the opposing "head block" signal at
> the far end of the single track, which immediately tumbles down all
> opposing automatic signals between the two head-block locations, thus
> establishing protection against head-on collisions.   So you get proceed
> signals in one direction, and stop signals in the other.  In other words,
> the entrance of a train into a segment of single track between head-block
> locations, set up the **traffic** between the head-block signals.  That was
> done by one relay at each end of the segment of track, which was usually,
> in my experience, nomenclatured as the "**F** relay (like the F  in the
> word traFfic, which is where the F-nomenclature came from.)  Signalmen
> generally refer to it as the "Follow Stick Relay," although I thought that
> term was a bit corny.
> <  Exception ... there are always exceptions to everything.  On the Punkin
> Vine, when no trains were between opposing head-block locations, the
> automatic signals  between those locations would stand at Clear in both
> directions.  When a train passed one of the head-blocks, the automatics for
> the opposing direction would tumble to stop. >
> It was the invention of ABP which made possible an effective system of
> using block signals to prevent head-on collisions on single track.  Before
> APB,  the real benefit of automatic block signaling had been for the
> prevention of rear-end collisions, and in the general economics of
> railroading, Automatic Block Signaling was generally employed in double
> track territory, in one direction only on each main track, e.g. eastbound
> automatic signaling on the eastbound main track, and westbound automatic
> signaling on the westbound track.
> Now, if you look at everything that happened in the world of signal
> engineering between about 1910 and about 1919, you see why the N&W was
> galvanized into making some quantum leaps in signaling during this period.
> About 1910-1913, the AC track circuit is introduced and becomes popular,
> largely doing away with primary batteries along the lines, so the N&W
> strings 440 on its own pole line and goes to AC track circuits and signals
> (but switches at ends of double track, and at passing sidings, are still
> hand operated.)   Just shortly before this, the Upper Quadrant semaphore
> was invented, and it had an electric motor.  In 1914 the doublet signal
> lens was invented, and signal range improves markedly. In the late
> 19-teens, the prevision filament signal lamp is perfected, further
> enhancing signal range.  In 1919 or so, APB signaling is invented and
> marketed, and operations on single track Divisions are sped up
> noticeably.  Also in this time period, the remote control of small
> interlockings became possible (e.g. the remote control of a one-switch
> interlocking at Bluff, the east end of the Low Grade Tunnel, from Walton.)
> Shortly thereafter, the US&S Train Control Laboratory in Swissvale invented
> the Copper Oxide rectifier which made trickle charging of storage batteries
> possible along the railroad, and that was the death knell of the old
> primary batteries.  In the 1920s, the N&W began replacing Semaphores with
> Position Light Signals, and in the 1930s coded track circuits came out,
> and in the 1940s CTC begins showing up, and that brings us up to where
> things stood when I worked in train service on the N&W in the 1960s and
> 1970s.
> My guess is that the Bristol Line may have been the first single track
> territory on the N&W to get APB signaling, AC track circuits and AC
> semaphores. (Someone needs to work out the history of N&W signaling and
> signal installations.)
> The result was that by 1920, signaling on the N&W looked nothing like it
> had looked a decade years before.  Electric automatic block signals were
> now spacing the following moves of trains, and also protecting the head
> ends of trains against opposing movements on single track.  The train
> blocking function of the old time **Block Stations** along the line had met
> its demise.  What offices remained open handled Train Orders and perhaps an
> interlocking or two, but the automatic block signals were handling the
> **blocking** of the trains.
> I will attach a copy of S.N. Wight's patent for the APB track circuit,
> which has a drawing.  But you are on your own for figuring out the
> circuitry, as it has been thirty years since I **studied it out**  and paid
> attention to it in the field.
> You really should think about talking to a professional Signalman, for
> remember, as I told you, I am only an outsider to the topic.  A **casual
> observer,** no less.
> ----------  abram burnett  ------------
> Offering Master Classes in Turnips
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