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While browsing through the Peco track catalog, I came across this interesting turnout - the SL-84 which is described as a Catch (Derail) turnout. I was trying to fathom out what it would be used for as it only appears to switch one rail. Any ideas?
This is just as you said, a derail, and is usually found gaurding the main from a yard lead.
They are generally powered and have a flashing light indicating their position as well as a recorded message broadcast over the radio, at least the ones I've been thru.
I use these at my yard leads to keep operators from fouling the main, it's helped save a train or two in the past.
They are a safety measure used to protect the main line from yard tracks usually. They can also be used to protect swing and lift style bridges.
Like any derail on a siding, these are used to derail a train that is not supposed to be going through them at that time. A string of runaway cars would derail onto the ground before fouling the main or running of the end of the track if the bridge is up!
Used particularly at normally open bridges. It would be placed a few hundred yards from the end of track and be in the derail position until it was lined by the dispatcher after assuring the bridge is down. This type of derail is now mandated by FRA because of some really horrific accidents at open bridges. The NWP had one near Schellville, California about 20 years ago. A bascule type lift bridge was mistakenly in the up position with the counterweight across the tracks. A local freight was given the green by mistake in foogy weather and crashed head on into the counterweight, crushing an SD-9 and killing the entire crew. If the signal was tied into a derail switch and positive indicator at the the dispatcher's panel that the bridge was down, the accident could have been prevented. It happened in our sheriff's department's jurisdiction and I was one of the first deputies on scene after hiking about half a mile down the track, since there was no road access. I've seen a bunch of accidents, including plane crashes, but this was about the worst I've ever seen in person.
