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

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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


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                  Sent to You from my Telegraph Key
Successor to the MAGNETIC TELEGRAPH LINE of 1844
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Moderator:
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