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Thread: Modern railroad signals

  1. #41
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    Wow, I don't mean any disrespect. But I think, in all fairness to everyone else on the forum, you should refer signalling questions to professionals.

    Quote Originally Posted by pbender View Post
    Most signals in the US today are automatic signals ( varriations of either ABS or APB ).
    Not sure what you mean by "automatic." Certainly ABS or APB are, but manual interlockings/control points are not until a train clears the OS.

    Quote Originally Posted by pbender View Post
    When the signals are controlled by a tower operator or the dispatcher through CTC, the operator can only set the signals to a more restrictive setting than track conditions allow. This means a dispatcher can set a route through an unoccupied switch or set a turnout to stop that might otherwise display proceed, but not vice versa.
    Not certain what you mean here.

    Here's what happens: In CTC territory dispatchers/control operators can clear a signal over a route at all manual interlockings under their jurisdiction. First they line the switches and derails -- if any. Once the switches/derails are in correspondence (the request -- normal or reverse -- matches the indication -- normal or reverse) the DS can clear a signal over the requested route. The signal system will not clear a signal over a route through which a switch is out of correspondence, a track circuit in the OS in the route is showing occupancy, and if the approach track past the leaving signal is occupied.

    The DS does not select the aspect -- that is determined by the relay logic/equation logic from inputs the plant is receiving from the location in advance. These inputs come in the form of coded DC through the rails (coded track circuits), or DC via pole line be it aerial, buried copper, or digital data via fiber optics.

    Intermediate signals always have number plates indicating milepost and track. They are not controlled by the DS and are simply Automatic-Permissive Block signaling. The input they receive from the location in advance determines their aspects.

    And that's all CTC is: Manual interlockings (control points, OS's, etc.) with APB signals in between.

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    Quote Originally Posted by TwinDad View Post
    My guess would be, having rambled for four paragraphs now... that each company provides the other direct "taps" (for lack of a better word) into the other company's track occupancy sensors, for however far up/downstream is necessary to provide the required information (likely just in the immediate vicinity of the junction). Access to and maintenance of these "taps" would I presume be via some form of agreement between the companies that is mutually beneficial (tit for tat, quid pro quo, share alike, etc. "you show me yours...")
    I'll use a very recent example. It happened on March 26.

    Union Pacific has trackage rights on the BNSF from CP Tukwila to CP Argo on BNSF's Seattle Subdivision. The UP main enters/exits BNSF rails at CP Tukwila.

    In order for the Seattle Terminal dispatcher to line a UP train off Main 1 at CP Tukwila and onto the UP main, or vice versa for the UP dispatcher, our signal bungalow and the UP signal bungalow have to be communicating the right data back and forth. What's communicated is track occupancy, signal aspects, home signal status and some other data. We have a vital logic controller interfacing with a simple four-wire Bell 212 modem that transmits and receives data with the UP's Bell 212 modem and VLC on the other side of the tracks.

    If, like what happened last summer, the UP main in advance of Tukwila is showing a track occupancy due to a switch circuit controller going out of adjustment due to the heat, then our DS can't line a UP train off Main 1 onto the UP -- can't get a signal to clear into a red block. Vice versa if something's haywire at Tukwila.

    Or, like what happened Tuesday, if my logic controller ain't talking to the UP's, nothing can happen. We were receiving but not transmitting. Bad logic processor which was easily swapped out.

    Even then the UP DS can't just run a train onto the BNSF willy nilly. Our DS has to first line the switch and clear the junction signal at Tukwila. We can run a UP train off Main 1 onto the UP though without having the UP DS lining a signal. But the two DSs do call one another..

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    Thanks for all the updates clarifications and corrections, Paul. Nice to hear how it works from someone in the biz.
    Never mistake a guy who talks a lot for a guy who has something to say...

    CH&FR Site and Blog: http://www.chfrrailroad.net and http://blog.chfrrailroad.net
    Appalachian Railroad Technology: http://www.apprailtech.com


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    Quote Originally Posted by pbender View Post
    True, when two railroads cross. Where two railroads link to oe one another, there could be other arrangements going on ( trackage rights, for example ) because the rail system only works when different carriers interchange cars.



    This is all done with track circuits.

    When a train moves from one railroad to another, someone ( probably the train crew ) will contact the dispatcher asking for permission to enter his track. You will hear these conversations if you listen to the radio traffic. In the case of the interlocking plant in this thread, The two signals on the crossover between NS and CSX are the ones used to convey authority to proceed onto the other line.

    Paul
    No, it's really not all done with track circuits. Please see my response above to TwinDad's questions.

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    It's nice to have you on board Paul seeing as you do this kinda stuff everyday and can answer our questions with some authority.

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    Quote Originally Posted by TwinDad View Post
    So... the signals around the area tap into whatever track circuits they must in order to figure a correct (safe) aspect, regardless of the track circuit's owner (again I assume some business agreement that allows this... it would in fact be mutually beneficial, after all)...
    The signal control system and the track circuits are separate. The heart of a track circuit is the track relay. It has contacts through which low DC voltage passes to other relays which affect signal control and signal lighting, or as analog inputs into an analog-digital converter in a vital logic processor. Or in the case of a coded DC track circuit, the relay is internal to the logic controller.

    That's the only way that signals and the track circuits interface -- via the track relay.

    From there, either the signal control and lighting relays control the signal aspects, in the case of analog relay logic, or the vital logic controllers do through vital input/outputs and signal drivers.

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    Quote Originally Posted by TwinDad View Post
    Thanks for all the updates clarifications and corrections, Paul. Nice to hear how it works from someone in the biz.
    You're welcome. I hope it's not too much "Inside Baseball," you know, Sabermetrics-like stuff.

    The one caveat is that some other carrier's might do things a little differently. But I'm thankful that in my time with BNSF that I've been able to work with a wide array of manufacturers' equipment that some guys outside the Seattle area have never gotten to work on because their territories are rather homogenous. We have something of everything around here.

    I got here just before the last of the old relay-logic houses were replaced. That's where you really learned about signaling.

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    If I come off kinda abrupt, it's not meant that way.

    Remember, Michael, when we put the old MILW signal up in Dave's yard -- January 1999? I should have hired on then!

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    Quote Originally Posted by pbender View Post
    All turnouts in signalled territory are connected to the signal system in that if a turnout in a block is thrown, it causes a short circuit. This short circuit is the trigger for a block to show occupied ( equipment wheels also cause the block to be occupied because they short circuit the track ).
    OK, that's not correct either. There is no short circuit associated with lining a switch in a block in ABS or CTC territory. There are shunt breaks (opens) in ABS territory. And in CTC territory how, if the DS lines a route but a track indication appears due to a "short" would the DS ever be able to clear a signal over the route? The signal logic simply won't allow a signal to clear into an occupied track.

    When a switch in ABS territory is lined away from the main, contacts on a switch circuit controller are opened. A SWCC is just a big, heavy four-pole, double-throw switch. Move the switch points by 1/4" and the FRA says those SWCC contacts better break open or else.

    In most cases, DC voltage passing through the SWCC contacts is sent to a switch repeater relay. When the SWCC contacts open, the repeater relay drops, and in turn DC energy from its contacts is removed from the home-distant relay, causing the signal to go red. An occupied track circuit also de-energizes the H-D relay via the track relay contacts opening and interrupting enery to the H coils, setting the signal to red. In other cases, the SWCC is wired a shunt break which drops a track relay when opened.

    Quote Originally Posted by pbender View Post
    In practice, electronic locks are installed on any manual turnouts within interlocking limits for several reasons.
    Not sure what you mean by "several reasons." There are only a couple. Electric locks are used in CTC territory and are considered signals. If a power-operated switch machine with a manual interlocking control point with signals is not used for entering/leaving dark territory, then a manual turnout it used. With a manual turnout in CTC territory, movement from dark territory into CTC territory is controlled either via a leaving signal or an electric lock.

    Quote Originally Posted by pbender View Post
    As stated earlier, a train* may occupy the siding for switching work or so the dispatcher can get it out ofthe way. The electronic lock will also be in place to prevent the position of the turnout from changing under a train ( which also forces the train crew to be a certain distance from the turnout before changing the switch position ).
    Quote Originally Posted by pbender View Post
    The operator ( or dispatcher ) may also have control over when the switch is unlocked, to force crews to keep him informed of the crews actions.
    Any train crew or maintenance personnel must by rule call the dispatcher to get permission prior to operating an electric lock. Even then, the relay logic will make a check of the approaches to the lock to ensure there is no oncoming traffic before unlocking the lock.

    In other words, the DS can say OK for a train crew to operate the EWL to come out onto the main, but even just physically requesting the lock -- you have to turn the handle turn part way to request an unlock -- turns adjacent signals red. The system will run time -- very short if nothing is on the approach to the lock; or several minutes if something is on the approach and to give time for trains to stop safely. When time expires the EWL unlocks and moving the handle the remainder of the way physically lifts the plunger/pin that drops down into a hole in the throw rod when the switch is locked.

    If a train wants to exit the main into dark territory, there is a quick unlock feature. Certain positioning of the train is required to active an AC overlay circuit ahead of the switch points. Permission from the DS is still required.

    In other words, the system is designed to get a train off the main ASAP, but makes checks if a train wants onto the main.

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    explained very well, and very interesting reading, paul. thank you

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    Quote Originally Posted by Paul Schmidt View Post
    It's not an absolute short circuit. The nominal resistive value of what we call a "shunt" is .06 Ohms and it's a federal law. All cars and locomotive are to have that nominal Ohmic value. The shunts we use in the field to test track circuits and crossing island circuits are also rated at .06 Ohms.
    Technically, you are correct. From the physics point of view, the currentis divided between the relay and the shunts accross the rails. Because the path through the shunt(s) is physically shorter in length, there is insufficient current to keep the relay open, so it falls to a failsafe position.

    Paul

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    Quote Originally Posted by Paul Schmidt View Post
    Wow, I don't mean any disrespect. But I think, in all fairness to everyone else on the forum, you should refer signalling questions to professionals.
    Remember, there are different points of view as to how things work. A signal maintainer may think of things differently than a dispatcher does. That doesn't make a dispatchers comments in how things work wrong.


    Not sure what you mean by "automatic." Certainly ABS or APB are, but manual interlockings/control points are not until a train clears the OS.
    At the controller points, the dispatcher sets a route and the signals clear once the conditions permit. The signal acts like an automatic signal once the route is set ( especially if fleeting is enabled ).

    Here's what happens: In CTC territory dispatchers/control operators can clear a signal over a route at all manual interlockings under their jurisdiction. First they line the switches and derails -- if any. Once the switches/derails are in correspondence (the request -- normal or reverse -- matches the indication -- normal or reverse) the DS can clear a signal over the requested route. The signal system will not clear a signal over a route through which a switch is out of correspondence, a track circuit in the OS in the route is showing occupancy, and if the approach track past the leaving signal is occupied.
    The dispatcher can actually set a lot of this information in advance, and hit the code button. The field equipment does the required actions when conditions permit.

    The DS does not select the aspect -- that is determined by the relay logic/equation logic from inputs the plant is receiving from the location in advance. These inputs come in the form of coded DC through the rails (coded track circuits), or DC via pole line be it aerial, buried copper, or digital data via fiber optics.
    The DS specifies the direction of travel at a control point. When the DS hits the code button, this tells the interlocking plant to allow traffic in the specified direction when track conditions permit. The dispatcher's direction of travel switch is a three position switch. In the center position, traffic is not allowed in either direction, even if track conditions would permit the movement.

    We describe this action as the dispatcher setting the signal to a setting more restrictive than the track conditions permit.

    Paul
    Last edited by pbender; 29th Mar 2013 at 02:31 PM.

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    Quote Originally Posted by pbender View Post
    At the controller points, the dispatcher sets a route and the signals clear once the conditions permit. The signal acts like an automatic signal once the route is set ( especially if fleeting is enabled ).
    That's right, Paul, and now I understand better the perspective of what you wrote earlier. Once the home signal at a control point is requested or cleared by the DS, its aspects can upgrade as traffic in advance of the home signal passes distant signals when fleeting is in use. (Fleeting is a bit of a pain to maintainer, because just when we think we'll get time on a track in the plant "after the next train clears," well, sure enough, the block remains occupied. There's no track indication, but we can clearly see that the DS has keep the route in for another train.

    Quote Originally Posted by pbender View Post
    The dispatcher can actually set a lot of this information in advance, and hit the code button. The field equipment does the required actions when conditions permit.
    That's right. I have access to the same screen the DS does via my laptop, and it's very interesting to watch his chess game as he sets up routes that might not clear for several more minutes. The requests are "in" as we say, but the won't clear until, as you say, conditions permit. In signal logic, the conditions are ANDs and ORs involving locking, tracks, switches, and signals, and can be traced out from a print or by looking through the logic equation books.

    Quote Originally Posted by pbender View Post
    The DS specifies the direction of travel at a control point. When the DS hits the code button, this tells the interlocking plant to allow traffic in the specified direction when track conditions permit.The dispatcher's direction of travel switch is a three position switch. In the center position, traffic is not allowed in either direction, even if track conditions would permit the movement.

    We describe this action as the dispatcher setting the signal to a setting more restrictive than the track conditions permit.
    Thanks for clarifying that and it makes perfect sense now about the three-position switch; that's from the Old School dispatching machines which sadly have almost all but gone by the wayside, replaced with modern dispatching stations. The DS today has a graphic represenation of the entire territory under their purview (which might require several computer screens), and just points and clicks at the routes and signals with a mouse these days. The signaling programming prevents the DS from lining conflicting routes.

    I appreciate the perspective of someone from the dispatching side of things, Paul

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    Quote Originally Posted by Paul Schmidt View Post
    I appreciate the perspective of someone from the dispatching side of things, Paul
    My perspective is actually of a software developer. I don't have any experienced as a dipacher on the prototype, but I have sat in the dispatcher's office and asked lots of questions.

    You need to understand how the parts of the system interact to be able to write software that emulates what happens on the prototype.

    The track and signal circuits work differently on a model and the prototype, but the principles of the dispatchers interaction with the system are the same.

    Paul

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