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Thread: A good read on train authority

  1. #1
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    Default A good read on train authority

    Dave Husman over on the MRH forum has written a concise, easy to understand primer on what authorizes a train to move. He briefly goes into various methods of operation (TT/TO, CTC, TWC, etc.). It's not written in "jargonese"; it is written, though, by a former UP officer who knows what he's talking about.

    If nothing else, if you thought "yard limits" has anything to do with a yard, you'll learn why it doesn't.

    One thing, when he gets to Automatic Block System signaling, he uses the abbreviation "APB" but doesn't define it. That's Absolute-Permissive Block signaling, a robust form of ABS. From a signaling engineering standpoint, I find APB to be the most intriguing and more complex than CTC*. While pure ABS provides protection primarily for following moves, APB provides protection for following and opposing moves -- and does so without a dispatcher clearing absolute signals.

    Not much APB remains on the BNSF. Until a couple years ago, APB was the signaling mode on the Oregon Trunk. Now CTC, with its PTC overlay, is taking over. Yard limits in the Northwest are less frequent as well.

    *When I was working in May 2010 as a relief signal maintainer on the OT, I got a trouble call re: a train encountering two yellow signals in approach to a green at an absolute signal at North Paxton with no train ahead. So essentially the absolute signal was telling the two signals in approach that its aspect was red, not green. System was designed for double yellows in approach to a red at the absolute signal account the grade.

    This is where I learned about the Green-Yellow Repeater relay at the absolute signal. The G-YPR is a slow-release relay that replicated the position of the searchlight signal vane, which would be either full left or right depending on track occupancy of the blocks beyond, or dead-center red with the block ahead occupied. When the relay coil was energized due to a green or yellow aspect, it closed a contact that sent voltage down the pole line to the two intermediate signals in approach. Those signals would then display something more favorable than yellow.

    But a mouse had chewed through one of the G-YPR coil energy wires; the relay had dropped and was not reliably conveying information. So, fail-safe being what it is, the APB signal system told the train crew to be prepared to stop at the next absolute signal.

    Easy fix, but I learned a lot troubleshooting it.
    Last edited by Paul Schmidt; 15th Jun 2019 at 11:46 AM.

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    That's interesting Paul. I was an Area Controller in New South Wales (Australia), kind of like a train controller/despatcher but with only CTC equipment. We had similar problems at one location with ants getting in the relay and stopping it functioning. Happened every autumn, with the first cold weather.


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