CPL signals and how they operate (NW Mailing List)
NW Mailing List
nw-mailing-list at nwhs.org
Thu Jan 23 22:57:24 EST 2020
Position lights weren't supplanted by CPLs. And we have to be careful with semantics here. What the N&W ended up with were "Colorized Position Light" signals. Most purists agree that Color Position Lights"those favored by the B&O are an entirely different genus. The N&W CedPL signals were an evolution. The first mutation in 58 only changed the Left and Right bulbs to red and extinguished the center bulb at the same time. The other two positions were still three fog penetrating yellow. It was several years later that Top and Bottom were changed to green. In both cases the signal heads weren't changed just the light bulbs. It was still later that the center bulb (only on top heads) was removed. There was logic work to turn the center bulb on and off before it was just removed.
The coded track system was used on the Shenandoah Div first as part of the cab signal and Automatic Train Control system. The tradeoff was that it could use the rails to convey occupancy information between blocks and reduce the number of pole line wires but with more relays and logic.
Rick C Stone
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