From: roma@ux1.cso.uiuc.edu (Jon Roma)
Newsgroups: rec.railroad
Subject: Re: Semaphore Signals
Date: 30 Aug 92 05:23:46 GMT
Distribution: na
Organization: University of Illinois at Urbana-Champaign

cal@ee.ryerson.ca (Calvin Henry-Cotnam) writes:

>In article <BtpDJy.CMB@ux1.cso.uiuc.edu> roma@ux1.cso.uiuc.edu (Jon Roma) writes:

>>This is an oversimplification but in signaled territory without centralized
>>traffic control (CTC), the default signal aspect is green.  In CTC territory,
>>the signals controlled by the dispatcher (i. e., the ones at the ends of
>>sidings) display red by default.  Intermediate block signals in CTC
>>territory are automatic block signals and are not controlled directly
>>by the dispatcher.   green is normally displayed in both directions on
>>intermediate signals when neither adjacent CTC control point is lined for
>>a train.

>In some situations this is true.  Here are two differing examples right
>here in Toronto:

>CPR - Belleville Sub - mile 199.5 (Kennedy) to mile 204.2 (Don Mills)

>On each of the two CTC controlled tracks, when the RTC (that's
>Canadian for dispatcher -- Rail Traffic Controller) lines a control
>point for traffic in one direction, the intermediate signals in the
>opposite direction tumble to red.  After a train has passed, and if
>no other traffic has been lined, signals in both direction cycle
>to thier most permissive aspect.  This is what was described above.

>CNR - Kingston Sub - Mile 325.2 (Scarborough) to Mile 321.2 (Guildwood)

>On each of the three CTC controlled tracks, when the RTC lines a control
>point for traffic in one direction, the intermediate signals in the
>opposite direction tumble to red, and remain that way until such a time
>that traffic is lined for thier direction.  The intermediate signals
>in the direction lined will cycle to red as a train passes then cycle
>back to its most permissive aspect as the train moves further down the
>line.  This differs slightly from the above description.

I omitted the latter scheme from my original posting for simplicity but
there are indeed a number of places where CTC works the way you described.

What's likely is that, in these territories, there is a notion of ``traffic
locking'' which demands that a direction of traffic be established for each
track between each CTC control point.  Lining a route in one direction
requires that a corresponding traffic direction first be established via a
separate lever.  Establishing a direction of traffic tumbles down the
opposing intermediate signals to red independently of whether or not a
route is established.

Before the invention of CTC, traffic locking was used to allow bi-directional
operation of multiple main tracks while every set of controlled crossovers
between mains was run from separate interlocking towers.  The tower operators
at adjacent controlled points were not able to line opposing trains onto the
same track by traffic locking between towers.  The traffic levers at two
adjacent towers had to correspond with each other before one of the two
operators could line a train into the tracks between the towers.  The
traffic locking circuit was broken through all the intermediate track
circuits, preventing the traffic direction from being changed while trains
were in the block(s) between tower.  Changing direction of traffic
required both operators to move their levers to the appropriate position.

Later schemes eliminated the need for a separate traffic direction lever;
the act of establishing a route itself prevented concurrent opposing moves
and is what triggers intermediate signal tumble-down in these implementations.

However, in a number of places where traffic locking was in use, the old
circuitry was retained when these towers were replaced by CTC.  These
locations would thus behave as you describe.  I don't know if this is
the case on the portion of CN's Kingston Subdivision that you mentioned
but it is plausible that this busy line became bi-directional at an
early date and retains some vestiges of the old technology.

The 20-mile long CTC territory on the Illinois Central between Otto and
Gilman, IL had traffic levers for every segment between control points.
In addition, this implementation was unusual in that each control point
had a set of controlled signals governing moves leaving interlocking limits.

>As described in the earlier post, the "most permissive" aspect for an
>intermediate signal depends on its location relative to the next CTC
>control point that generally stays at red until traffic is lined by
>the RTC.  I do say "generally" as some systems allow for automatic
>cycling at control points.  For example, at a location where several
>trains are to pass along the same route in the same direction, it is
>sometimes possible to line the route once and select the automatic
>cycling feature to eliminate the need to re-line the route for each
>train.  Thus the signals would appear to operate as the intermediate
>block signals as a train passes and moves down the line.

In the United States at least, this is referred to as ``fleeting'' and
is common on rapid transit systems and busy multiple-track main lines
where there are few divergences and where most trains follow a regular
routine.
-- 
Jon Roma
Computing and Communications Services Office,
University of Illinois at Urbana-Champaign
Internet:  roma@uiuc.edu      	UUCP:      uunet!uiucuxc!uiuc.edu!roma