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

Sun Jul 22 17:40:54 EDT 2018

At the risk of opening my mouth and revealing ignorance, I will be most appreciative if someone could provide or direct me to an education on cab signals. I have seen many references to them over the years but know nothing about them other than what I read in the recent posts.

Specific questions are:

Why were they used only on the Shenandoah Division?

What did they tell the engineer?  I had supposed they told what the indication(s)of the next signal were but the posts suggested perhaps something else.

What did they look like in the cab?

Why were they discontinued?

Why did ACL use them between Petersburg and Richmond for the N&W trains?

Any insight will be appreciated. I decided to finally ask.   Thanks.

    Ray Smoot

Sent from my iPad

On Jul 22, 2018, at 4:47 PM, NW Mailing List <nw-mailing-list at nwhs.org<mailto: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

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