[PRCo] Re: MetroRail Signalling
Phillip Clark Campbell
pcc_sr at yahoo.com
Tue Jun 30 14:01:49 EDT 2009
> From: Edson Tennyson <etennyson at cox.net>
> To: J Aurelius <jaurelius at centurytel.net>
> Cc: Phillip Clark Campbell <pcc_sr at yahoo.com>; alschneider2 at juno.com; rejmhj at netzero.net;
> j_swindler at hotmail.com; pittsburgh-railways at dementia.org; jack.may at americomm.net;
> gary-elaine at comcast.net; nawdry at bga.com; crvlkotula at aol.com; philgcraig204 at yahoo.com;
> billvigrass at verizon.net; bob.dietrich1 at verizon.net; bbente at bellsouth.net; trams2 at comcast.net;
> allmanr at verizon.net; shadow at dementia.org; akftrain at aol.com; miklosfrank at comcast.net; russell.jackson at stvinc.com
> Sent: Monday, June 29, 2009 8:17:12 PM
> Subject: Re: MetroRail Signalling
> AUDIO TRACK CIRCUITS
> Yes, John is right about that long time. About 60 years ago Robert Brown of
> Union Switch & SIgnal Company took me up into his room in West View, PA.
> where he showed me a circuit he had set up to register when PCC cars
> passed below his property on the #10-15 line of Pittsburgh Railways.
> Unknown to anyone but Brown (and now me) he had snaked two thin wires down the hill ,
> under the road and attached them to the car tracks to create an audio frequency circuit
> to register the passage of each car. Pittsburgh Railways knew nothing about it.
> He took what he learned to work at Union Switch & Signal ands got them into audio circuits.
> He parlayed that into a signal job with The Pennsylvania Railroad Company in their
> communications department. I visited him in Philadelphia about 1956 and
> in Paoli a few years later. He was very instrumental in creating the
> Pennsylvania Trolley Museum at Arden, PA, and died early from a heart attack.
> He taught me to operate P.R.Co car # 3753 which I had often ridden when I lived in
> Bethel Borough the Township. Bob Brown is well remembered I am sure
> and appreciated. I think he applied audio frequency to grade crossing signals.
> E d T e n n y s o n
Mr.Tennyson;
To use the old term: This is a hoot isn't it. PRC was an unwitting rolling
laboratory which contributed to the signaling system of today. It is a very
real kernel of history which will probably never make it into the history book
let alone the history of US&S.
Phil
Without a 'coast' but not a 'cause.'
-- --
"If thou wouldst rule well, thou must rule for God,
and to do that, thou must be ruled by Him ...
Those who will not be governed by God...
........will be ruled by Tyrants."
William Penn, founder of Pennsylvania
________________________________
On Jun 29, 2009, at 8:43 PM, J Aurelius wrote:
Audio frequency track circuits and their block boundary devices have been around longer than computers have, probably from right after World War II. The signal source can be an analog audio oscillator and the detector can be a relay. The fail-safe principle is still there as long as the frequency can't wander to that of an adjacent block: if the oscillator quits there isn't a signal to detect; if the relay fails it won't pick up (to announce a clear block).
>
>Signal systems that use computers are designed with considerable care using fail-safe principles.
>
>That said, track circuits of all kinds live out in a punishing environment (heat, cold, strong forces from track expansion-contraction and trains passing, vandalism) and block boundaries. They CAN fail to the unsafe side - an insulated joint can fail so it isn't insulated; an electronic boundary device can lose its integrity. Unsafe failures of signal systems ARE RARE but have always been with us. Unsafe actions by human drivers, while also rare, are a more common cause of accidents.
>
>John Aurelius
>----- Original Message -----
>>From: Phillip Clark Campbell
>>To: J Aurelius ; alschneider2 at juno.com ; etennyson at cox.net
>>Cc: rejmhj at netzero.net ; j_swindler at hotmail.com ; pittsburgh-railways at dementia.org ; jack.may at americomm.net ; gary-elaine at comcast.net ; nawdry at bga.com ; crvlkotula at aol.com ; philgcraig204 at yahoo.com ; billvigrass at verizon.net ; bob.dietrich1 at verizon.net ;bbente at bellsouth.net ; trams2 at comcast.net ; allmanr at verizon.net ; shadow at dementia.org ; akftrain at aol.com ; miklosfrank at comcast.net ; russell.jackson at stvinc.com
>>Sent: Monday, June 29, 2009 1:32 PM
>>Subject: Re: MetroRail Signalling
>>
>>
>>> From: J Aurelius <jaurelius at centurytel.net>
>>
>>> To: Phillip Clark Campbell <pcc_sr at yahoo.com>; alschneider2 at juno.com; etennyson at cox.net
>>> Cc: rejmhj at netzero.net; j_swindler at hotmail.com; pittsburgh-railways at dementia.org;
>>> jack.may at americomm.net; gary-elaine at comcast.net; nawdry at bga.com; crvlkotula at aol.com;
>>> philgcraig204 at yahoo.com; billvigrass at verizon.net; bob.dietrich1 at verizon.net;
>>> bbente at bellsouth.net; trams2 at comcast.net; allmanr at verizon.net;
>>> shadow at dementia.org; akftrain at aol.com; miklosfrank at comcast.net; russell.jackson at stvinc.com
>>> Sent: Sunday, June 28, 2009 4:21:21 PM
>>> Subject: Re: MetroRail Signalling
>>
>>
>>> Audio-frequency (hundreds of hertz) track circuits are common on modern rapid transit systems.
>>> These frequencies aren't good for railroad track circuits (except as an overlay to control
>>> grade crossing gates) because they don't propagate down the track far enough for the
>>> 1 mile plus railroad signal blocks. On rapid transit lines, blocks typically are 500-750 feet long.
>>> With audio frequency circuits it isn't necessary to cut the rails and provide a path
>>> (impedance joint) around the insulated joints for traction power return, as was used
>>> on older systems. The device at the block boundary keeps the audio energy from
>>> moving into the next block and injects energy at a different frequency into the nextblock.
>>> The energy from the first block is detected by a relay (or other device) that works
>>> only from the first frequency - if it's present it has traveled down the block without
>>> being shorted out by a train; if it isn't present the signal system decides the block is occupied.
>>
>>> This is probably over-simplified but I think it's right in principle.
>>
>>> John Aurelius
>>
>>
>>Mr.Aurelius;
>>
>>
>>Thank you very much; I now understand the basic concept.
>>
>>With the old relay system, fail-safe was built into the system. 'A' weak
>>link in this system was the human element; operators could miss the signal
>>or otherwise ignore same.
>>
>>It now seems 'computers' or electronic circuitry somewhere takes the
>>place of the human operator, even to the point of not recognizing occupancy
>>and thus resulting in an accident. I hope a successful resolution is found
>>soon.
>>
>>
>>
>> Phil
>>Without a 'coast' but not a 'cause.'
>>-- --
>>"If thou wouldst rule well, thou must rule for God,
>>and to do that, thou must be ruled by Him ...
>>Those who will not be governed by God...
>>.......will be ruled by Tyrants."
>>William Penn, founder of Pennsylvania
>>
>>
>>
>>
________________________________
>>
>>----- Original Message -----
>>>From: Phillip Clark Campbell
>>>To: alschneider2 at juno.com ; etennyson at cox.net
>>>Cc: rejmhj at netzero.net ; j_swindler at hotmail.com ; pittsburgh-railways at dementia.org ; jack.may at americomm.net ; gary-elaine at comcast.net ; nawdry at bga.com ; crvlkotula at aol.com ; philgcraig204 at yahoo.com ; billvigrass at verizon.net ; bob.dietrich1 at verizon.net ;bbente at bellsouth.net ; trams2 at comcast.net ; allmanr at verizon.net ; shadow at dementia.org ; akftrain at aol.com ; miklosfrank at comcast.net ; jaurelius at centurytel.net ; russell.jackson at stvinc.com
>>>Sent: Sunday, June 28, 2009 11:58 AM
>>>Subject: MetroRail Signalling
>>>
>>>
>>>Mr.Alan Schneider;
>>>
>>>
>>>Thank you very much for this report. Thank you to the many contributors on
>>>this subject. The news reports 'seem' well done and I was able to glean
>>>much information from them; comments from contributors underscored these
>>>observations and brought much clarity to this disaster. The postings have been
>>>most productive -- 'mostly.'
>>>
>>>I have changed the subject line and have questions to seek a greater understanding
>>>of the signaling system. I found the URL for the included article:
>>>http://www.fredericknewspost.com/sections/news/display.htm?StoryID=91910
>>>because I wanted a closer look at the included photo:
>>>http://www.fredericknewspost.com/photos/09/06/26/91910_large.jpg
>>>
>>>It is already established that 'computers' handle the signals through 'ATC;' while I
>>>understand the concept I am not familiar with the details. My reference is the US&S
>>>signals as used on PRC interurbans. Rails were gapped and insulated; an impedance
>>>bond allowed ground return around the insulated rails while keeping the signal circuit
>>>current within the block.
>>>
>>>'Parallel' construction is evident in the MetroRail photo but the rails are not gapped and
>>>insulated are they. Please note the dual set of cables between the rails in each track
>>>that arc through a half circle before attachment to each of the rails; this 'suggests' an
>>>insulated gap in the rails which, again, is not clearly evident. There are far more cables
>>>between the rails than the parallel US&S impedance bond.
>>>
>>>Also noteworthy is the large gap in the 600-volt(?) 3rd-rail. As the cabling between the rails
>>>of each track seems identical and the 3rd-rail is gapped on both tracks I am 'assuming' that
>>>such a gap in the 3rd-rail exists wherever such 'signal cables' exist. Is this true? Why is
>>>it necessary to gap the power supply at signal cabling locations? Why aren't the running
>>>rails gapped, or are they? I assume each truck has a 3rd-rail shoe on each side; it then
>>>'seems' that current is never disconnected from the train as the gap 'seems' shorter than
>>>truck spacing, or is it? The right hand track has ties that extend further than standard
>>>'apparently' to accommodate 3rd-rail in the gap area; the same may be true of the other
>>>track but the ties could be covered by the ballast.
>>>
>>>If someone is able to give some clarity to the computer style ATC signals circuits it will
>>>be most appreciated. Thank you to one and all for such efforts.
>>>
>>>
>>>
>>> Phil
>>>Without a 'coast' but not a 'cause.'
>>>-- --
>>>"If thou wouldst rule well, thou must rule for God,
>>>and to do that, thou must be ruled by Him ...
>>>Those who will not be governed by God...
>>>.......will be ruled by Tyrants."
>>>William Penn, founder of Pennsylvania
>
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