CPL signals and how they operate

NW Mailing List nw-mailing-list at nwhs.org
Tue Jan 7 09:32:48 EST 2020

It appears there may be at least a little general interest in the details
of how the CPL signal system operated so let’s start with the basics.  To
determine when there is equipment in a block, the rails of the block are
electrically insulated/isolated from those of the adjacent blocks.  The
rails would need to be cut at the appropriate points and perhaps insulating
material placed between the ends.  How were tie plates and joint bars
insulted?  A DC voltage was applied to the rails, positive to one and
negative to the other.  I believe the rail voltage was set such that a
small current would flow through the coil of a relay to keep it energized
when the block was unoccupied.  However, when any wheel set entered the
block a conductive path was created from one rail through the wheel on that
rail, through the axle, and through the other wheel to the rail.  This path
“shorted” current around the coil of the relay and caused the relay to
de-energize.  (Is this the correct operations?)  I would like to know how
this relay actuation was used to cause the block signal to display the
“stop and proceed restricting” aspect which for CPLs would be two
horizontal red lights.  That takes care of the block that is occupied, but
occupancy information has to be sent to the preceding block signal so that
it displays the “approach” aspect, two amber lights at slant 45 degrees
clockwise from vertical on CPLs.  I assume (and we all know how dangerous
assuming can be) that some signal was sent from the occupied block
equipment to the preceding block signal equipment by means of the pole
line.  I would very much like to know the details of how that information
was encoded and transmitted, and how the equipment of the preceding block
used it to set the proper signal aspect.  Well, that should be enough to
get the ball rolling for now.  Thanks in advance for any and all additional
information and clarifications.

Jim Cochran
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