Signaling Beyond Simple Passing Sidings

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BigDumbDinosaur
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Re: Signaling Beyond Simple Passing Sidings

Post by BigDumbDinosaur » Mon Dec 11, 2017 9:30 pm

ChuckHackett-844 wrote:When you get into bi-directional running, I assert that ABS/APB signals are not sufficient without requiring someone to back up in the case of APB conflicts (e.g.: meets at intermediate signals - not desirable at a large meet). Further, managing traffic flow and preventing deadlocks requires using a) a dispatcher, b) timetables, c) track warrants, etc. (which is not practical for 90% of ride-on railroads).
This is the case in full-sized practice. If a train doesn't have authority to occupy a track it cannot be moved, even if the nearest facing signal is CLEAR. APB prevents conflicts that arise when train movements are not to schedule, especially when opposing movements are involved.

APB in riding scale is doable, but will in practice be unworkable unless a club is very serious about formal dispatching and enforcing operating rules. A big part of the problem is most riding scale hobbyists may know about railroads and trains, but little about railroad operation and practice. This can range from simple things like failing to close a turnout after setting out a car, to serious issues, such as not understand and/or obeying signals. The latter, in particular, illustrates something that another Chaski user has frequently noted: a signal system is only as good as those who operate on the signaled track. If train operators can't be relied upon to obey signals then riding scale APB doesn't have a ghost of a chance of being successful.

Anyhow...
apb_no_trains.gif
Typical APB Territory
For the benefit of readers who may not be familiar with railroad signaling, above is an illustration of a stretch of APB territory track, with no trains present. Odd-number signals face west-bound (WB) trains and even-numbered signals face east-bound (EB) trains. Both sets of signals are affected by movements in either direction. With no trains in the area, all signals will be displaying CLEAR aspects.

Signals 1, 42 and 51 are referred to as "headblock signals" and are "absolute," which means these signals cannot be passed while displaying a STOP aspect, unless authority to violate the signal is granted by dispatch. All other signals are permissive, which engineers may treat as "stop and proceed." In this case, "proceed" means to run at restricted speed (10-15 MPH or thereabouts) and expect to encounter another train, or a track defect that caused the signal to display to STOP. Incidentally, the acronym APB (absolute permissive block) comes from this mix of absolute and permissive signals.

Note the presence of passing tracks, which allow meeting trains to pass or overtake. Despite the positions of signals 2, 41 and 52 relative to the passing track entrances, once a train is completely in the passing track it is no longer under signal protection. Operating rules dictate how trains are to be moved once off the mainline.

In the next illustration, a WB train has passed signal 1, at which point it has entered the west-bound headblock. Block occupancy detection is bi-directional so the APB can tell which way a train is going. This feature is signified by the "cut" through the track midway between signals.
apb_eb_train.gif
East-Bound Train Entering Intermediate Track Segment
Now the opposing signals 12, 22, 32 and 42 have all "tumbled" to STOP. So far, so good. However, here is where the deadlock Chuck mentioned can develop.

Suppose that right after the WB train passed signal 1 an EB train passes signal 52 and its engineer having misunderstood his train orders, fails to take the passing track. The EB engineer will see signal 42, located at the east end of the passing track on which his train is supposed to be, change to STOP. However, he's highballing with 100 loaded coal hoppers and can't stop before reaching the signal. This being the case, he will pass signal 42 and occupy the east-bound headblock, causing opposing signals 11, 21 and 31 will tumble to STOP. The resulting signals status would be as illustrated below.
apb_opposing_trains.gif
Opposing Trains Pass Headblocks
Needless to say, both trains must immediately be stopped. Assuming the engineers are alert and promptly react when they see their facing signals change to STOP, the EB train will stop before reaching signal 32 and the WB train will stop before reaching signal 21, this being the worst-case scenario. Although their respective signals are permissive and hence can be passed if safe to do so, the two engineers will see each other at this point and will know to not proceed. The line is deadlocked and dispatch would have to determine which train is superior and instruct the other train's engineer to back down and take the passing track. In practice, this seldom happens on full-sized railroads, as they don't operate with anywhere near the train density seen at some Live Steam clubs. However, should it occur, the risk of a cornfield meet will not exist, as long as both engineers follow the rules and obey the signals. Of course, should one train experience a brake problem all bets would be off.
How (as an example) would one signal a Wye using only ABS/APB signals to support bi-directional running? (for complete description of the requirements see my initial post in this thread and the clarifications that followed - the diagram, etc.)
I can't answer that without doing a full analysis of all possible movements. As time permits, I will see what I can conjure.
Science makes it known. Engineering makes it work.

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johnpenn74
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Re: Signaling Beyond Simple Passing Sidings

Post by johnpenn74 » Sat Jun 16, 2018 5:37 pm

Chuck,
Reading an old post I am curious what you figured out on the WYE Signaling.

Here's my .02. I agree with a lot of previous points that APB wired in this case will simply give you an identical permits on both sides of the wye. I beleive the solution is would to treat the whole thing at an interlock plant and go from there. That doesn help in live steam.

I would follow another prototype example that I have seen at a diamond. This particular diamond always had an approach signal a half mile back that was yellow. That is, if you got to the diamond and the opposing train pulled over you would be red and have your proper notification to stop from the previous signal. I would do something similar here.

As fore mentioned the failing here is operational. Both trains get a yellow or a green (based on whether the approaches are looking 1 block or to the next block) and end up meeting in the middle. I believe the solution is like our diamond 1) always default the signals to red and 2) have the relays evaluate the request based on SIDING occupancy (both tracks must be observed from other side of wye) Or request segment or Request button.

Of course the problem this creates is the default red might be inconsistent with signal aspects depending how you request blocks.

JP
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Erskine Tramway
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Re: Signaling Beyond Simple Passing Sidings

Post by Erskine Tramway » Sun Jun 17, 2018 12:06 pm

BigDumbDinosaur wrote:
Mon Dec 11, 2017 9:30 pm
Suppose that right after the WB train passed signal 1 an EB train passes signal 52 and its engineer having misunderstood his train orders, fails to take the passing track. The EB engineer will see signal 42, located at the east end of the passing track on which his train is supposed to be, change to STOP. However, he's highballing with 100 loaded coal hoppers and can't stop before reaching the signal. This being the case, he will pass signal 42 and occupy the east-bound headblock, causing opposing signals 11, 21 and 31 will tumble to STOP. The resulting signals status would be as illustrated below.
apb_opposing_trains.gif
Needless to say, both trains must immediately be stopped. Assuming the engineers are alert and promptly react when they see their facing signals change to STOP, the EB train will stop before reaching signal 32 and the WB train will stop before reaching signal 21, this being the worst-case scenario. Although their respective signals are permissive and hence can be passed if safe to do so, the two engineers will see each other at this point and will know to not proceed. The line is deadlocked and dispatch would have to determine which train is superior and instruct the other train's engineer to back down and take the passing track. In practice, this seldom happens on full-sized railroads, as they don't operate with anywhere near the train density seen at some Live Steam clubs. However, should it occur, the risk of a cornfield meet will not exist, as long as both engineers follow the rules and obey the signals. Of course, should one train experience a brake problem all bets would be off.

I had a situation like that one morning. the Crew Caller called and said: "How soon can you get to Edgemont?" "About 45 minutes." "OK, you and a Brakeman will ride out to East Owens with the Roadforeman to relieve a crew".

The Eastbound Engineer had 'blown' his stop at East Owens, and had all five motors and five cars out through the power switch onto single track. The Engineer claimed he'd misjudged his stop, but my theory always has been that his buddy, the Conductor, was running. I figure that, account when we swapped head end crews, the head Brakeman had to go back to the waycar to get his grip, which tells me he wasn't riding the motors like he was supposed to be. Fortunately, the westbound meet was just sitting there on the main, back from the other train, and they never came close to getting together. The westbound Engineer tried to con the RFE into letting him dogcatch the eastbound train, and giving his train to me, so he could go right back home instead of Gillette. The Roadforeman wasn't having any of that :D We ended up cutting off the cars through the switch, so the Section could fix it. Then, the Dispatcher 'talked us by' the Absolute Signal westbound, ( "Extra 5500 East has permission to pass the signal displaying Stop indication at East Owens westbound from the main track to the main track.") and we backed in the clear and met the west bound fleet.

So, stuff like the illustration happens on the 'big railroad', same reasons, human error.

Mike
Former Locomotive Engineer and Designer, Sandley Light Railway Equipment Works, Inc. and Riverside & Great Northern Railway 1962-77
BN RR Locomotive Engineer 1977-2014, Retired

Liesegang
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Re: Signaling Beyond Simple Passing Sidings

Post by Liesegang » Fri Jun 29, 2018 6:12 pm

At Maricopa Live Steamers, there is a simple scheme to avoid the difficulties talked about in previous posts.

On the subdivisions that are signaled, the inbound train has priority (steamers might need water since the subdivisions are long). Each subdivision track structure is a series of doubles track segments connected by single track segments. Right hand running, each double track segment is unidirectional for each track and each is considered a main track. The logic is simple to implement and works well with a high number of trains in operation (we have three highly attended meets a year).

The entire signal system on each subdivision has no software, everything is done strictly with hardware logic. Easy to use and easy to repair if needed. We use the three color system - Stop, Approach, and Clear.

As long as the engineer pays attention to what he or she is doing, everything works fine. If one engineer runs a red light and causes a conflict then the two engineers involved figure out what to do. You just can't fix stupid sometimes, even with duct tape!

Terry

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