Signals at interlockings

[NW Mailing List]

Signals associated with interlockings are normally required to be able to
display aspects beyond the capabilities of automatic block signals.  Let’s
take a look at a simple example of the siding shown in the figure:



West End
___________________________________________________________________  East
End

                            O
\
                 /

                            O
   \
                  /

                             |
       \
                /



\-----------------------------------------------------------------------/




*                             *

          O


*                            *
                                  O
     O



                     O
      |



                       \___/




         |







An Eastbound train at West End can proceed to East End via two different
routes:  the main line (indicated by the solid line) or the siding, which
diverges from the main line (indicated by the dashed line).  First we’ll
look at what is needed for the West-facing signal at West End.

If the West End switch is lined so the train will go “over” it and stay on
the main, then the West End signal needs to be able to show the normal
block signal aspects (clear, approach and stop and proceed).

 If the West End switch is lined so the train will go “through” it and take
the diverging route into the siding, then the West End signal needs to be
able to show the diverging approach aspect.  You probably wouldn’t want a
train to travel above medium speed on the siding track, so there probably
is no need for the signal to display the diverging clear aspect.

Finally, for the case where there might be a Westbound train that is going
to hold the main while the Eastbound “goes in the hole” but the West End
switch hasn’t been properly lined yet, you’d probably want the West End
signal to display the stop and stay aspect.

Don’t want these posts to run too long, so I’ll cut this one off and follow
up with another about the West facing signal at East End.

Jim Cochran
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