How does it actually work and why such a complex design when 3 color signal like used today is much simpler?
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Leaving the Isle of Denial briefly, I was fortunate enough to play with the BNSF signal trailer at National Train Day. The searchlight mechanism is not really more difficult than a 3-light signal. Basically, relays control the aspects as they do with a 3-light signal.
By default, the signal indicates RED so that if there's a failure in the system, the signal indicates stop (at least until the batteries die). There is a pendulum mechanism that holds the three colored internal lenses. The red is in the center as this is the non-powered position of the pendulum. For green, an electro-magnet is energized one side; for yellow one on the other side is energized. To conserve bulb and potentially battery life (I'm pretty sure they're switching to bright-white LEDs by now) the signal goes dark if no train is present in the approaching block. On the subject of batteries, they're charged by conventional power going to the signals.
As to why, that's pretty much seems up to the individual railroad. Santa Fe seemed fond of them; UP seems to like their vertical 3-light signals (the new ones look like a Darth Vader helmet); Metrolink uses the vertical 3-light; I've seen both types on SP. I don't see the searchlight units as being significantly more difficult to maintain.
In a modeling context, you can make searchlight signals using two-lead bi-color LEDs will allow a 3-color aspect by feeding positive DC, negative DC, or AC voltage through the leads. You only need some SPDT relays and an AC power source.
Hopefully "Laidoffsick", "Hot Water" or "Number 90" will chime in on what I missed.
The big selling point of the searchlight signal was the compound lens that allowed the use of a low-wattage bulb while projecting a bright light toward an approaching train. In the late 1920's, this was really leading-edge technology.
However, in the modern era its Achilles' heel is the fact that it has one moving part as described by Matt.
Thanks for the info. An interesting type of signal for sure. I guess we could have the same conversation about "doll arms" and why they are no longer used...unless they are already in place.
Originally Posted by Number 90:
The big selling point of the searchlight signal was the compound lens that allowed the use of a low-wattage bulb while projecting a bright light toward an approaching train. In the late 1920's, this was really leading-edge technology.
However, in the modern era its Achilles' heel is the fact that it has one moving part as described by Matt.
That explains why they don't seem to be proliferating like the three-light signals. I had no idea they dated back to the 1920's, especially given that semaphore signals remained in use for so long.
Mike, look up a video on youtube called "The Railroad Signal". This was produced by the New York Central, and at about the 6:45 mark, you can see the operating mechanism of a searchlight type signal.
