Signals: What I don't know . . .

[NW Mailing List]

As the saying goes, What I don't know about signals would fill volumes.  So
I would like to throw out the little bit that I think I know and hopefully
y'all can correct my errors and elaborate of the things that are basically
correct.
It seems to me that there are three types of signals:  block, interlocking,
and the "glue" signals that connect the blocks to the interlockings.  Basic
block signals seem fairly straightforward. They have one arm with a number
plate below it.  The arm can display only "clear" when the next two blocks
are unoccupied, "approach" when the second block in advance is occupied,
and "stop and proceed restricting" when the next block is occupied.  So the
only thing that determines the aspect that these signal present is the
state of block occupancy detectors for the next two blocks.  This doesn't
seem too complicated, but like everything else, special cases arise to
muddy the waters a bit.
For example, suppose you're pulling a heavy coal drag up a 2% grade and
your locomotives are bout on their knees.  What you would not want to
happen is to encounter a signal at "stop and proceed restricting", bring
your train to a full stop only to see the signal go to approach a minute
later.  For one thing you might not be able to get your train started again
or at a minimum it might take a lot of time and effort to get her going
again.  So the way I understand it, to cover such situations, the
"restricting" aspect can be added to a block signal so that the train only
needed to slow to restricted speed (15 mph) instead of stopping.  I think I
remember seeing that this could be accomplished by the addition of a circle
"G" (grade) emblem under the arm as well as, I suppose, by adding a second
arm with only two positions on the reverse diagonal.

Conversely, if you're coming down a steep grade rather quickly with that
same heavy train pushing you, you might need a significant distance to
bring her to a stop.  In some cases where blocks are short, it might take
more than whole block to do this.  In such a location, if you were to enter
a block at speed, whose signal displayed "approach", you might run past the
next signal at "stop and proceed restricting" before getting her completely
stopped.  I believe this would be the motivation for "advance approach"
aspect.  I would assume this would be done with a second arm with two
lights on the diagonal that would come on whenever the top arm went to
"approach".

As was stated in one of the notes in the safari thread, going from
signalled to "dark" territory was indicated by "restricing".
Any other "block" signal aspects, special circumstances/situations?  I'll
start on interlocking signals in a later message.

Jim Cochran
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