N&W Train Control, Shenandoah - John Rhodes' Question

[NW Mailing List]

Abram,

Thanks for the history lesson, I really appreciate it.  I work for a
railroad that still uses coded track circuits and I am decently well
acquainted with them.

John Rhodes

On Sun, Jul 22, 2018 at 4:46 PM, NW Mailing List <nw-mailing-list at nwhs.org>
wrote:

> John Rhodes asketh:
>
>
>    "Did the signal system use the PRR type pulse cab code system from
> union switch and signal?"
>
> Mr. Rhodes --  The answer is, No.  As explained in the articles, the N&W's
> Shenandoah Division cab signal and train control system worked by steady
> (not coded) AC energy in the rails.  The key was that there were two AC
> inputs into the rails, and they were out of phase with one another.  A real
> jackass way to work, but that was the technology of the 1920s.
>
>
> The principle of "coded track" did not come around until the following
> decade.  The first installation of coded track was in 1934, again at
> Lewistown, but this time on the 4-track Main Line.  As  used there,  the
> cab signal system consisted of AC energy pulsed into the rails at three
> different code rates, and picked up inductively by a detector bar ahead of
> the first pair of wheels on an engine.  At first, the old DC steady-energy
> track circuits were kept, and the AC cab signal current was just
> superimposed on top of the DC.  The next innovation was to "code" both the
> DC track circuit energy and the AC cab signal energy.  Finally, they found
> out they could do away with the DC energy for the track circuit, and
> operate both the track circuit and the cab signal system with pulsed
> "coded" AC pumped into the rails.  This concept was marketed by Union
> Switch & Signal" under the name "Universal Code," probably because it would
> do everything the railroads needed."
>
>
> Don't be mystified by all this talk of "code."  If you tap a live wire on
> to a rail180 times per minute, you have just created "180 Code" and that
> says to the signal system, Give the train a Clear signal.  Do it 120 times
> a minute, and you have created "120 Code" and that says to the signal
> system, The third block ahead is occupied, so give the train an Approach
> Medium.  Do it 75 times a minute and you have created "75 Code," which says
> to the signal system, The second block ahead is occupied and the next
> signal is at Stop, so give the train an Approach.  Take away all coded
> energy from the track, and the signal system says, Something is wrong in
> the block ahead, so set the wayside signal to Stop and give the most
> restrictive aspect on the cab signal.
>
>
> Creating the various code rates is rather simple and is done with nothing
> more than electro-mechanical relays.  However, DE-coding the code rates at
> the far end of the block is the gimmick, because that job requires
> resonantly tuned track transformers, and you need one for each of the code
> rates.  They are big and weigh about 50 pounds each (lots of iron and
> copper,) so I never bought any home.
>
>
> Attached is a photo of some code transmitting and code receiving relays.
> LEFT to RIGHT in the photos:
>
>
> 1.  An old 1920s type NF crossing flasher relay, which has nothing to do
> with the cab signal relays, so overlook it.
>
>
> 2.  The 75 Code "generating" relay... actually it is nothing but an
> electro-magnet oscillator which clicks back and forth 75 times a minute
> based on simple magnetism and a make/break contact.
>
>
> 3.  The 120 Code generating relay.  Works just like the 75 Code relay.
>
>
> 4.  The old "black box" is an example of the  original 1930s code
> equipment, but works with "mainsprings" wound around a reciprocating,
> oscillating spindle, magnetically actuated.  I use it to get 180 Code.
> (And yes, it was so old I had to tear it apart and re-build it.)
>
>
> 5.  A "CD" or Code Detecting relay, which picks up the pulses of energy
> ("code") from the track and sends them to the RTR (resonant track
> transformer.)  It will follow any of the three code rates.
>
>
> 6.  A type P-4 relay, which was used for many, many functions in signal
> coded signal systems, from track circuit energy to CTC
> control-and-indication applications.  The contact carrier is vertical, so
> that the effects of gravity are minimal when the relay is following a very
> weak electrical signal.
>
>
> The little "green dots" above several of the relays are just little
> ceramic "Transorbs" (transient voltage suppressors, or "spark killers" in
> the vernacular.)  Their principle use is to prevent the inductive kickback
> from collapsing force fields in the relay coils from sparking and chewing
> up contact metal.  And quite frankly, I have now forgotten just why I
> installed them on these particular circuits !
>
>
> The little flat plate in front, with one rotary switch and three micro
> toggle switches, is just the control to turn off/on my cab signal system,
> and make it perform in various ways.  (There are seven cab signals on the
> wall at the "other end" of the system.)
>
>
> In today's world of this PTC garbage, all old electro-magnetic equipment
> of this nature is redundant and quickly headed for the scrap heaps.   With
> the "electronic track circuits" which came into vogue in the late 1980s,
> everything is done with "RFs,"  very low voltage AC energy at "radio
> frequencies" put into the rails.  I hope the Roanoke Transportation Museum
> (yes, I like old names) will ask for a set of the old coded track signaling
> equipment before it is all gone.  It would make a great display and is
> pretty simple to hook up.
>
>
> Also attached is a second photo, which shows the "other end" of the
> system, where the 4 volt AC control voltage actually makes the cab signals
> work.  The square black blocks are ersatz "AC relays" with a little
> rectifier and an opto-isolator inside, and can use a 2 volt input to switch
> a 15 amp, 220 volt load.  Pretty amazing.  They came from the ceiling
> lighting system when the old Altoona Train Dispatcher's Office was closed
> in 1990.  The wooden box was the signal maintainer's "bulletin box" which
> hung on the wall in old AR Tower at Gallitzin.  At left is another P-4 code
> detecting relay.  The little indicator light above it is a
> toilet-needs-dumping indicator LED array off an NS engine.  Everything in
> this photograph was junk, and the only thing I had to buy was the little
> brass door hook on the front.
>
>
>            -- abram burnett
>
> ancient order of barnyard mechanics
>
>
> ===========================================
>                   Sent to You from my Telegraph Key
> Successor to the MAGNETIC TELEGRAPH LINE of 1844
> ===========================================
>
>
> Moderator:
> https://www.nwhs.org/mailinglist/2018/20180722.
> 2012_07_19_039_Cab%20Signal%20CT%20Relays.jpg
> https://www.nwhs.org/mailinglist/2018/20180722.
> 2012_07_18_037_Cab%20Signal%20Display.jpg
>
>
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