CPL signals and how they operate

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Continuing to try to assimilate previous information, John Marbury caused
me to re-examine my analysis of the article on coded track signals.  The
article emphasized that the new system

accomplished all control by means of the track circuits without any line
control circuits.  It also stated the track circuits were fed by
transformers with interrupted 110V code signals being applied to the
primary winding.  What the article failed to mention was the turns ratio of
the transformers and thus what voltage was applied to the track itself.
John said this most probably would have been in the couple volt range
consistent with previous systems.  The article did mention that an in-line
variable reactance was included in the transformer secondary circuit in
series with the track to provide adjustment as follows:  “The track
circuits are so adjusted that a minimum axle current of two amperes flows
when a track circuit is occupied; this current is adequate to operate cab
signal apparatus.”  Since the wheelsets provided a short circuit path from
rail to rail, I suspect the cab signal apparatus would have inductively
sensed the magnetic field produced by the 2A current flowing through the
rails at the coded pulse rate.

And speaking of the coded information, as Larry pointed out the track
circuits described in the article only carried coded block occupancy
information (180 ppm for next two blocks unoccupied or 75 ppm for next
block unoccupied) along with turnout configuration (set for main or
diverging route).  Nothing about how to display this information was
included in the code.  Logic within the signals used the coded information
to determine which lamps to activate.

To be continued . . .

Jim Cochran
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