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Thread: American safety systems modeling

  1. #11
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    Quote Originally Posted by Vince View Post
    Nice quote but the cab signals in the PRR MP54 displayed FIVE aspects, the bottom was the 'no-code' display of stop.
    Indeed, there are five lights, but per this blogger and this historian, they were only capable of displaying four aspects. Three pulse codes, plus "Restricting," the lack of a code.

    Quote Originally Posted by Vince View Post
    Ever ridden in the cab of a GG1, an MP54? I have . . .in both.
    No, I have not. And I concede I have no data or experience regarding the PRR's equipment. After all, that's why I asked...

    However, I have ridden the new SEPTA trains with railfan windows. Here is one such video. Notice the engineer doesn't get the downgrade (turn up the volume and you can hear the warning tone) or commence braking until he actually passes the signal.

    Quote Originally Posted by Vince View Post
    In all cases my in cab signal observations were as I stated; "the cab signal displays the oncoming signal NOT the one just passed".
    ...And I am very interested to hear that, and I am not discounting your experience. But, I have two rival train simulators and people who are very familiar with contemporary Amtrak operations telling me that the MSTS way is wrong, and that the cab signals should reflect the state of the block you just entered. So who's right? Perhaps both of you are. If we gather more information, I am sure we could reconcile these conflicting interpretations.

    Quote Originally Posted by Vince View Post
    Think about this for a minute, or maybe you'll need a bit longer longer: Why would you ever want a display of what you just passed.
    I can think of a few reasons. So that if the engineer misses the signal head due to fog or snow, or forgets the current one in force, he can be informed. So that if traffic conditions improve, he can receive the upgraded aspect immediately (which does happen with my script).

    P.S. Kindly refrain from making jabs at my reading comprehension.

    Best,
    Ryan
    US-based railfan and programmer. Author of various timetables and safety scripts for Open Rails.

  2. #12
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    Quote Originally Posted by dcarleton View Post
    Also remember that the signals protecting the block that your train is currently in should always indicate "stop".
    Of course. Cab signals don't constitute a license to ignore the lineside ones. Actually, that would be quite impossible, with only 4 pulse codes to represent all 12 possible aspects...
    Ryan
    US-based railfan and programmer. Author of various timetables and safety scripts for Open Rails.

  3. #13

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    We know that. It's just that the cab signal system cannot continue to read the signal that the train just passed, since that signal went to stop as soon as the train passes it!

  4. #14
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    The script records the state of the signal at the exact moment the train passed it. So, even after the signal drops to Stop, the train can still remember the aspect in force.

    In reality, I believe, the pulse code signaling system would transmit the code for the incoming block, and as soon as the train enters that region, its electronics would pick up the code. As in that SEPTA video.
    Ryan
    US-based railfan and programmer. Author of various timetables and safety scripts for Open Rails.

  5. #15
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    I'm no expert, but my understanding of cab signals is that they tell you the state of the next signal, and is primarily a supplement to (sometimes a replacement of) wayside signals. The cab signal telling you the state of the block you're in would mean it would always show either Stop or Restricting, since the block your train is in will by definition always be occupied. If something happens ahead of you and the signal ahead drops to stop, the cab signal will probably do so, as well (not guaranteed - not all systems continuously updated). A rail break might drop the cab signal, as well, depending on how the pulse code is sent (probably through the rails). But wouldn't the primary warning about the state of the block the train is currently in have come while the train was still in the preceding block, and was still facing the signal protecting said block?

  6. #16
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    I think the correct way to frame this problem is in terms of blocks, not signal heads.

    Code:
       Stop              Approach             Approach               Stop
    (was Clear)           Medium
        o                   o                    o                    o
        |  [===][===][===]  |                    |                    |  [===][===][===]
    ----+-------------------+--------------------+--------------------+------------------->
                Clear          Approach Medium          Approach              Clear
                                                            |---------|
                                                            Restricting
    Cab signals indicate the current state of the block, which can change instantaneously. Signal heads indicate the status of the upcoming block, and regulate movements between blocks.

    This is why cab signals do not show the "next" signal.
    Last edited by rman; 05-17-2020 at 06:46 PM.
    Ryan
    US-based railfan and programmer. Author of various timetables and safety scripts for Open Rails.

  7. #17

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    Okay then, we'll consider this in terms of blocks. The block that you're in cannot be clear because it's occupied - by you!

  8. #18
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    I would argue that something along these lines is more likely:

    http://www.lundsten.dk/us_signaling/...cab/index.html

    While PRR may not have used the same logic (I have yet to meet a signal system from the NE US that was fully logical), that does seem the most likely setup

  9. #19
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    That's a great find, and it supports what I've been saying! The PRR system wasn't identical, but it was built by the same vendor, US&S.

    Notice that the cab signal remains at Clear all the way until the first Approach signal. It does not inform the engineer of the next signal in advance.

    Ryan
    US-based railfan and programmer. Author of various timetables and safety scripts for Open Rails.

  10. #20

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    Sure about that? In the Illinois Central example, in the event that the next block ahead of the train suddenly became shunted (by a malfunction or by human error), the next signal ahead would go to red, the AC pulse in the rails would be interrupted, and the cab signal would go to Restricting. How is that not informing the engineer about the next signal ahead?

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