[PRCo] Re: Derail

[Mark McGuire]

 And I was going to ask about the exit southbound(uphill) throught the tunnel but this explains it. Thanks Phil. I guess a novice streetcar operator had to learn where to take the power off or on to either coast or diverge. Were the interurban lines diverges and the 42,38, and 39 lines straight? I suppose one had to be coasting so as not to diverge onto the 44, 48, 47 trackage. Were there 3 switch points exiting the tunnel? Forgive me as I know basically nothing about operating a streetcar, but do so wish to learn.

                           Mark

-- Phillip Clark Campbell <pcc_sr at yahoo.com> wrote:
The TTC moved their switch points using a system known as necessity action didn't they.  That term seems applicable to the Pgh derail as it is necessary to take action to change the derail point.  If necessity action is official terminology then your description would seem to be valid.

There may be several types of necessity action.

Many or most systems used power application for diverge (right or left) with coast for straight through.

If I understand TTC and also SLPS correctly their points remained in position unless action was taken to change the position so a streetcar could coast into a right or left turn if the point is already in that position.

Boston and DC saw all turnouts as a wye-type formation with two legs - power the right leg and coast the left or was it the other way around.  Regardless, there is a straight leg on both a right and left turn and to go straight in both cases would require power on one and coast on the other.

TTC is necessity action to change the position of the point otherwise coast.

Pgh is necessity action to make a diverge move otherwise coast..

Boston and DC are necessity action to take the right [or left] leg of a turnout otherwise coast.

All the above could use the power apply (toggle on a PCC;) the wayside electrical schematic to accept the power or coast application would be different.

I recently found the name of the simple overhead contactor that is often used in the Nachod signalling system which I can't remember can I.  I did a google search and found a couple references to Canadian patents which aren't online because they are too old and another reference to a German manufacturer of transit equipment but they didn't make the contactor.  Ohio Brass apparently didn't make hardware for operating track switches did they as I have never seen such in their catalogs.  Finding the correct terminology today will be difficult unless museums have that information in their files.

Only one contactor was used to set the derail; another contactor was used to reset the point to derail after the car passed.

Pgh had not a few locations where two contactors were used to set a point.  I do not have the official railway explanation but here is how it worked.  The operator would take the necessary action of either coast or power in the first contactor, usually the simple contactor like used in the Nachod system, and the second contactor was the standard switch contactor where the operator could either coast or use power and the point would set according to the action taken in the first contactor.  Looking at an example might help.

This system was used at Dawn jct where the 42 left the 38 outbound.  The 42 was US&S protected from Dawn to street paving in Beechview both directions so Dawn was probably interlocked.  An outbound 38 or 39 could get a red board because of an inbound 42, the 38 could proceed through the first contactor by coasting and then stop to allow the inbound 42 to clear.  When the 38 applies power it is too close to the second contactor so it goes through with power on but the switch sets for straight through.  Without the first contactor the 38 would be in trouble.

But this begs a question doesn't it.  Why not just move the one switch contactor back to prevent this problem?  Answer: a second car can move in and move the point before the first car has cleared with disastrous results.

Dawn Jct, Brookline Jct, SHJ exit tunnel all had the double contactor..  Where the interurbans and Ravine St lines parted at SHJ had only one contactor - this is not unlike Dawn jct so why the difference.  Some of the intersections on Forbes and Fifth near Craft had this double contactor arrangement.  I am hard pressed to know the exact reason(s) why such was used - do you know?

My apologies for being so long winded but the operation of streetcars is very fascinating.

Phil

PS - this terminology reminds me of the US&S system on the interurbans where it is described as Absolute / Permissive Sub-Divided Blocks.  A block could be SHJ to Castle Shannon and that is obviously sub-divided.  Need to find my signal book for the accurate description of a block but those who model tend to think of a block as an electrically isolated section.  Some signals like red over red are Absolute Stop where permission is needed to violate while other reds are permissive and may be violated after a certain time interval, and then only at speeds which would allow an immediate stop if the section is blocked.



----- Original Message ----
From: John Swindler <j_swindler at hotmail..com>
To: pittsburgh-railways at dementia.org
Sent: Friday, September 28, 2007 2:11:59 PM
Subject: [PRCo] Re: Derail

 
I once heard it referred to as a 'necessary action, delayed action' switch, or something like that.  
 
Was something like that an official PRC designation, or an explanation by on OT railfan to this (at the time) young kid railfan???
 
It was a long time ago.
 
John




       
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