CPL signals and how they operate

NW Mailing List nw-mailing-list at nwhs.org
Tue Jan 14 15:49:57 EST 2020

Tovarisch Burnett,

Thanks for the great response, a lot of information sparking many more
questions.  I found a copy of PRR drawing S-845:  Typical Circuits for
Absolute Permissive Block Signal System (attached).  I have been attempting
to understand parts of the system and would once again greatly appreciate
your help is sorting through the details.  I have been focusing on page 3
and specifically on the circuits related to signal 425.  There seem to be
six relays directly associated with this signal labelled “S”, “H”, “SA”,
“TR”, “ATR” and “ATRM”.  You have already explained the “track” TR relay as
being de-energized when any wheel set enters the block.

It looks to me like the block between signals 425 and 403 in this case
includes four electrically isolated sections of track and track relays 425
TR, 412 TR, 412 ATR and 403 ATR all provide indications of occupancy on
sections of the block.  What do the relay designations ATR and ATRM stand

I also suspect that the 425 H relay indicates occupancy for sections/blocks
in advance (is that the correct term?) of signal 425.  This is based on
contacts for those afore-mentioned track circuits being in series with the
coil for 425 H, but I can’t get a handle on how the “S” relays figure in.

It appears that the “SA” relay just provides a delayed version of “H” (why
was this needed?) and likewise that ATRM is just the opposite state of
ATR.  I’m sure I am missing some nuances here.

When it comes to lighting the lamps, it looks like in this scheme they were
only illuminated when the block section immediately in the rear (help with
the proper phrasing) was occupied.  One contact set of relay H was used to
select between 45, 46 or 90, 91, and 0, 1 were lit when the SA relay was

Are there any N&W signal circuit drawings of this type available?  I would
assume the systems were quite similar, but would still like to know details
specific to N&W.

I’ll cut it off here for this installment and keep studying the diagrams
hoping the lamp above my head will get a little brighter.
Jim Cochran

On Fri, Jan 10, 2020 at 8:06 PM NW Mailing List <nw-mailing-list at nwhs.org>

> Comrade Cochranovskyj, Excellency of the Holy PL’s,  and Accomplices :
> A few pointers to help you in your safari through signal-speak,
> interleaved with the answers to your questions :
> **H**  and **D**  in the vocabulary stand for Home and Distant.
> Undoubtedly, those were among the earliest of signal nomenclatures.   To
> understand them, project yourself back into the world of all those
> beautiful two-arm lower quadrant semaphores the N&W began installing in
> 1906. Each semaphore arm had only two possible positions:  horizontal, or
> (somewhat) vertical.  The first block beyond the signal location is called
> the Home Block, and the second block beyond the signal is called the
> Distant Block.  If the Home Block is occupied, the H Relay will be down and
> the TOP two-position lower semaphore arm will be at Stop.  If the Distant
> Block is occupied, the D Relay will be down and the BOTTOM semaphore arm
> will be at stop.  Sounds complicated, but if you understand it, you have
> the whole basis of block signaling in the palm of your hand.  Home and
> Distant are the building block concepts of automatic block signaling.
> **E**  in the nomenclature stood for Energy, and you are right to think of
> it as Voltage.
> **N** is for Negative.  Everything in early signaling was DC, so **N** was
> an intuitively obvious nomenclature for them to use.
> **R** for Relay.  Some railroads, in their designations, did not use an
> **R” suffix behind the nomenclature of relays.  The N&W did use the **R.**
> Since you are looking at PRR Standard Plans, realize that the **R** will be
> missing from relay names.  In those PRR Standard Plans, when it comes to
> nomenclaturing individual wires in a signal lighting scheme, **R** stood
> for **Red** on those prints… in other words, for Stop.
> What happened next is amusing…  AC current came into railroad signaling
> around 1910.  And railroad signaling needed nomenclatures for each side of
> the AC feed.  Early on, PRR nomenclature those as **ACB**  and **ACC,**
> meaning AC-Battery and AC-Common.  That shows the early signalmen were
> still interpreting AC current in the terms of their DC mentality.  I love
> it !  I have some of my own AC circuits nomenclatured ACC and ACB, just to
> honor the old ways. (We still had ACC and ACB nomenclatures in relay cases
> west of Altoona into the 1990s.)
> Eventually the industry practice came to be using **B12** for Battery 12
> volts, or B16, or whatever,  and **N** for the two sides of a DC feed, and
> E and N for the two sides of an AC feed.  AAR put out some simplified
> recommended practices and most roads seemed to follow those.  I think E and
> N are probably the current standards, but I make no effort whatsoever to
> keep up with the newfangled trendy  stuff.
> One thing I never understood was, when looking at transformer output
> terminals, how do you know which to designate as ACC and which as ACB?
> (Really, the only time this would become a practical problem is if you were
> seriesing transformer outputs.)  I asked all the best old time signalmen,
> and none of them knew.  So I asked them some signal engineers, and got no
> answers.  Finally I asked an old US&S signal engineer and got the answer:
> In manufacturing railroad signal transformers, the lead from the bottom
> layer of the coil winding is always terminated on the right binding post,
> and the lead from the top layer of the coil winding is always terminated on
> the left binding post.  So, for the man in the field, just arrange the
> connections so that the two transformer windings do not buck one another,
> and it really does not matter which you label ACB and which ACC.
> Your next question was how Track Repeater Relays are connected to the
> Track Relay.  Very simple.  The repeater relays have separate 12v supply,
> and a contact in the Track Relay just serves as a switch in the voltage
> supply.  Interesting tidbit you probably don’t need to know:  PRR
> nomenclature for a repeating relay was **M.**   Most other railroads used
> **PR** as the symbol for a Repeating Relay.  A Track Relay Repeater was
> usually **TPR.**   The N&W signal prints use PR for a repeating relay.  In
> really big interlockings, one repeating relay might not have enough
> contacts, so there would be repeaters of the repeaters, nomenclatured PPR,
> PPPR, PPPPR, and so forth.  US&S made a monster relay just for repeating
> purposes – they weigh about 20 pounds and have 8 Front and Back contacts in
> them, and if I recall correctly, the US&S catalog offered a 12 contact
> version of the same relay.
> In my experience, all **relay rooms** for electric relay interlockings ran
> at 12 volts DC.  That was just a good, common and easily available voltage,
> and allowed for using 1000 Ohm relays for the necessary functions of
> checking switch position, lever positions, and the like.  Sensitivity of
> the relay was not a major issue.
> Track Relays, on the other hand, needed to be VERY sensitive instruments,
> and what was called **shunting sensitivity** was their most important
> parameter.  They needed to shunt (drop) where required under any possible
> condition, and not remain up, which would allow for a switch to be moved
> under a train or a signal improperly displayed for the movement of a train.
> (Rusty Rail was the big bugaboo for track circuit safety:  rust acts as an
> insulator, and the very low voltage of a track circuit may not ionize the
> Ferrous molecules in the rust.)  For the reason of shunting sensitivity,
> the 4 Ohm relay was pretty much an industry standard.  The track circuit
> was fed off the battery bank and the track circuit voltage was adjusted by
> ceramic-wound slider resistors.  A typical track circuit with a 4 Ohm relay
> may operate at 3 to 5 volts, at about 0.250 amperes.  If you get interested
> in such things, there are many good books on track circuit adjustment, and
> you will learn a LOT by reading them.
> One more thing you asked about was the transmission of signal information
> along the track, from one point to another.  Originally, back when things
> were simple, all you needed was one pair of wires running on pole line from
> the battery end of the track circuit, to the relay end of the track
> circuit.  That provided you with binary information:  circuit either
> energized, or de-energized.  Someone very quickly realized that they could
> get a third bit of information while using just those two wires, by
> changing the DC polarity on that pair of wires on the pole line. So the
> combinations are then +/-,  -/+  and the complete absence of energy.  It is
> by using such **polarized line circuits** that Searchlight signal
> mechanisms work, giving three signal aspects with just two wires.  Very
> quickly, polarized line circuits were adopted for every possible function,
> as they saved the railroads a lot of Copper wire and a lot of pin positions
> on the pole line.  When polar line is applied to conveying track circuit
> information, the pair is called the **HD Pair,** because it communicates
> the condition of the Home and Distant blocks back to the block signal
> governing entrance to the block.  Using polar line, of course, means you
> need special relays which are sensitive to the reversal of line current.
> More information you will probably never use.
> To finish up on answering your question, in the 1930s it was realized that
> pulsed energy could be used to communicate many different pieces of
> information over a single pair of wires, or even through the rails
> themselves.  There were many permutations of this concept, and there were
> steady energy DC track circuits, steady energy AC track circuits, AC coded
> track circuits, DC coded track circuits, and pulsed AC and DC currents
> carrying information over line wires as well.  This, of course, required
> decoding equipment at the far end to translate the code rates of energy
> into a meaningful on/off signal. That decoding equipment was called a
> resonant track transformer, or a decoding transformer. This pulsed energy
> was the whole basis of railroad CTC and Cab Signaling.  A much more complex
> form of the same principle was Carrier equipment for telecommunications.
> Nowadays, all these information-passing functions are done by RF’s (radio
> frequencies.)  I think the Norfolk Southern still has in place a lot of its
> old microwave equipment, originally deployed by the Southern RR in the late
> 1950s, but the really modern way is to ship whatever information is needed
> either via fiber optics, or by the Ethernet.
> The N&W did not fool around and invested in the newest and best of signal
> equipment.  They were buying equipment for AC coded track circuits for the
> Norfolk Division back in the late1930s, right after that technology hit the
> market, and I understand that AC Coded Track system was still in place
> until the Southern arrived on the property.  BTW, the code rates generally
> used in the industry were 180 pulses per minute for Clear, 120 pulses per
> minute for Approach Medium, 75 pulses per minute for Approach, and absence
> of coded energy for either Stop or Restricting.
> At this point, I will probably be expelled from the List for
> loquaciousness.
> Confitior:  I was neither a Signalman nor the Son of a Signalman, so I
> offer up these answers on a Without-Recourse basis… which means you cannot
> sue me in a court of law if I got something wrong !  What little I know was
> learned by hanging around with the Signal Department as much as I could,
> going out on their big projects, asking the old heads a lot of questions,
> studying their prints and bringing home for play-purposes at least one of
> everything they threw away.  At heart, you know I am still just an old……
>  Turnip Farmer.
> -  abram burnett
>      << PS - **Confitior** = Latin for **confession.** >>
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