Glue signals (NW Mailing List)

[NW Mailing List]

Jim Cochran wrote:

> alignment/occupancy.  I would assume that if the interlocking were aligned

> for you to hold the main and there were no other trains, then the

> corresponding distant signal might show clear. 


Either clear or 'Advance Approach' depending on block length and max.
speed of the line segments.


> If the home signal was at

> stop and stay, would the distant signal show approach?  What about other

> cases?


The previous intermediate signal will show approach if the home signal was
either 'Stop and Stay' or Restricting (the previously discussed 'Approach
Restricting' aside), at least on current ex-N&W territory.


> I would assume that a distant signal might be considered a controlled

> signal since its aspect is determined by the state of the corresponding

> home signal which is in turn determined by the route set by the traffic

> controller.


No, a controlled signal is only one that is directly controlled by the
dispatcher. Note that their 'control' is limited mostly to lining switches
and requesting a permissive (non- stop and stay) signals in a direction
through the plant.  The dispatch does not specificaly set the actual
indication to clear or approach or diverging approach. Those field
indications are still automatically determined by the on-site plant
electronics. In fact, a dispatcher can actually request, for example, a
westbound signal through a plant even when the next track block west of
the plant is still occupied, presumably by a previous westbound train.
In that case, unless the signal can display a restricting into that track,
the signal will continue to display 'stop and stay' until the train clears
the block. The controlled signal will then automatically (without any
dispatcher intervention) change to approach, until either the first train
clears the second block (and the signal indication automatically changes
to Clear), or the second train passes the approach signal (when the signal
will again automatically change to stop).

In fact, the dispatcher's control is limited to requesting actions from 
the 'control points'. It is up to the on-site electrnics to decide based
on switch positions and block occupancy when and if to grant the request.
If you listen to radio transmissions between the dispatcher and trains,
you should never hear the dispatcher tell the train they have given them
a clear signal, because the dispatcher can't be certain from his or her
computer screens what actual indication the field signal shows (even if
they can make a reasonable educated guesss, they don't know for certain).
The most you will hear is phrases like 'proceed on signal indication' or
'be governed by the signal at ...', or 'indicates ready for you at ...'
(meaning the dispatcher requested the signal, and the signal sent a
positive response back -- but if the field signal is displaying stop,
the crew still needs to stop short.



Joe Shaw
Christiansburg, VA
http://www.krunk.org/