Glue signals

[NW Mailing List]

Previously we looked at automatic block signals that provide the engineer
only with information about occupancy of the next two blocks.  Signals at
interlockings, in addition, supply information on the route the train is
going to take.  Now what happens when these two systems meet?  For safety,
you need to give the engineer some information that requires a different
type of signal with some special aspects.

Suppose you, as an engineer, are looking at the last automatic block signal
prior to an interlocking that has been lined for your train to take the
diverging route.  Further assume that there are no other trains in the
area.  The automatic block signal could only show clear, because there are
no occupied blocks in front of you, so you would proceed past the signal
doing your maximum allowed speed (let’s say it’s 60 mph), not knowing the
state of the interlocking.  Now suppose that according to the TT, the
maximum allowed speed on the diverging route was slow (15 mph), you would
come barreling into the interlocking with its home signal showing diverging
clear.  If you didn’t have it memorized, you’d quickly grab your TT and
find out about the 15 mph speed limit on your new route.  Now you’d need to
try to reduce your speed from 60 mph to 15 mph in what would most probably
be an insufficient length of track.  This could be a problem, and the
motivation for distant signals that provide a safe transition from
automatic block signaled portions of the road (intermediate signals) to
interlockings (controlled signals).

These distant signals add a second arm and can display aspects that provide
information on the state of the upcoming interlocking.  For example, they
could display “approach diverging ( / over |) telling the engineer to
“Proceed preparing to take diverging route beyond next signal at prescribed
speed”.  This would give you time to consult your TT if you needed to and
slow your train if required for the diverging route.  I don’t know what
aspects would be displayed for other conditions of interlocking
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.  If the home signal was at
stop and stay, would the distant signal show approach?  What about other
cases?

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.

There is a lot of supposition on my part presented here.  Please comment,
correct and expand it as much as you can.

Jim Cochran
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