the basic question is: with automated signalling, are the signals controlled by both block occupancy and turnout positions?
From reading Railroad Operations and Railway Signalling, I understand that railway signalling circuits do implement digital logic (i.e. AND, OR) both to determine train direction and from that the logic governing signals (i.e. following vs. opposing signals). (The recent RMC article on signal left me wanting).
part of my understanding is that some signalling is just trying to prevent trains from occupying the same block (with obvious consequences) and is primarily a fail-safe.
But i’m thinking about how it could be completely automated.
I’m thinking about a multi-block section of single track between two sidings. The typical situation is as a train exists a siding, signalling logic can cause all the signals in the opposite direction to become stop and the signals in the direction of the train to providing following behavior, approach in the following block and clear in the next block.
but i’m wondering about the situation where there are trains at both sidings trying to enter the single track between them.
Would they both see clear signals? (what happens if they both exit there sidings at the same time)?
Or would the signalling logic (e.g. west end) recognize that the turnout (e.g. east end) is routed to a block (i.e. siding) occupied by a train and present a stop signal (e.g. west end)?
If there are trains at both sidings with the turnout routed to enter the siding, would each train see a stop signal an
With bidirectional signaling, as soon as one train left the block and entered the single track, all the opposing signals would ‘tumble down’ to red to block any opposing movements. If somehow both trains tripped the first block in at exactly the same time, there would still be several reds between each one so without blatant disregard for signal indications, a collision should not happen. There are other rules governing train movements as well, if this is not CTC. If this area IS CTC, then the dispatcher can only clear a movement in one direction at a time, so the one train would see clear and the other would still see stop. The first signal to enter the single track woudl always be an absolute, not permissive. Intermediate signals may be permissive to prevent a train from being stuck in the middle of nowhere in the event of a signal failure - detection failure should always drop all signals to red, and a signal failure (not lit, or maybe some vandals stole the semaphore arm) is always interpreted as the most restrictive aspect.
This is a simplification but should get the idea across.
Ok the first thing is all signaling and switches are interlocked to prevent opposing signals being cleared and so you can’t have two opposing trains occupying the same block section
If they both left the siding at one time one has had a SPAD signal passed at danger the driver will be stood down and could loose his job.
For following trains the signals should keep the trains two signals apart, not allways practical on a model railway.
Unless a very long distance which would be rare the gap between one siding and the next is only one block section crossing loops on single line working are spaced at regular intervals to facilitate the most practical and smoothest running of trains possible.
Yes the system does know which way the switches are set at each end of the block and gives the driver the apropriate signal indication red for main green or yellow for the loop or vice versa as the case may be and also if you are trying to send the train into an occupied track
An Automatic signal system is perfectly possible on a Model Railroad including the manual control and semi automatic parts of the system.
You just need to know the operating requirements of the operations division of the railroad then design and build the system acordingly.
Its all a question of how much work you are prepaired to do and how much it will cost you to do it and are you willing to make that spend.
If a signal goes black out it is considered to be red and the modern systems cascade acordingly and no train can pass it without train control’s permision
The older electric semaphore and colourlight systems did not nesasarily cascade
as such but they still went back to red even though there is no visible indication
But the same rule applied its red so don’t go passed it without train controls permision and the train drivers quickly let you know when something is amiss.
so a signalling system requires both block detectors and a mechanism indicating turnout position. I know that there are contacts on Tortoise switch machines that would serve this purpose, but i’m not familiar with any mechanism for turnout position indication with manually controlled turnouts without powered frogs.
I assume this also includes my question about the turnout position at the siding if it is routed to an occupied block.
I’m confused by the following image which illustrates my point. This suggests that signalling is a fail-safe since it seems that it would allow two trains to enter the single track from opposite directions, but stop short of entering the same block.
I’ve suggested that all the eastbound signals should indicate stop to prevent a train from entering the single track from the siding on the west because of the turnout routing at the east siding.
Chubb describe Visual Basic programming to support signalling in RMC. Are people really doing this? (i think encoding signalling logic in code as opposed to data is naive).
First off that diagram is rubish all the east bound signals should be red it shows opposing signals and on a real signal system that would never be able to happen
Unless there was what is the most serious and fortunatly increadably rare type of failure a wrong side failure
In signal teritory all switches driven, manual or electric switch locked have detection iether built in to the equipment or as a seperate attatched piece of equipment this tells the system if the switch is set normal reverse or not properly set.
block detection can be of two types ocupation indication with an alarm that tells you a train is entering the block at which end this is the older one
The modern one does the above and automaticaly sets up the whole block for the direction of travel as well.
You have two choices as to how it is all going to work one with old school electronics and relays with relay logic.
Or the modern way with programing of some sort signaling basicaly works on a yes no on off coding system all a bit beyond my knowledge base.
But people with the nessasary knowledge base would have used the RMC artical to program thier signaling systems that way.
A glitch in the code can give some shall we say very interesting results so make sure it is dead right.
Having a 1 or a 0 in the wrong place could be catastrophic.
And make sure all required Train movements, signal indications and switch positions are covered including the wierd one that may be required at location X.
The part that is the painfull bit is it is not unusual for each location or stretch of line to have its own version of the code, to match its individual operations division requirements so the chips can’t be swapped around.
If a switch requires a permision and a release from the train controler this is also some how in the coding so the system behaves the way it should, some things get very complicated and are well and
In ABS territory, in theory both trains might see clear signals if they simultaneously reached the entrance to the single track section. But that would mean that at least one of them was committing a serious rules violation, or that the dispatcher had made a disastrous mistake when issuing the running orders. In CTC territory the circuitry would only permit one signal to be clear.
Often there would be at least two intermediate signals between passing sidings and they would provide an unexpected restrictive indication, so the trains should still be able to stop before colliding. But with only one intermediate, or none, yes bad things could (and on rare occasions did) happen.
In addition in single track ABS signal indication does not convey authority. If a train has a clear signal that doesn’t mean it can go. If a train has a red signal that doesn’t mean it has to remain stopped. The authority is conveyed by the train orders, track warrants or block authority, not the signals. So for two opposing trains to leave the sidings at the same or any other time would first require a rules violation from the train crews or dispatcher. Not saying it can’t or doesn’t happen, but to have it happen first requires something else to fail.
You are assuming the dispatching system knows what the train is going to do, you are assuming the train in the diagram is going to continue. Both are bad assumptions. In ABS the signal system only knows the positions of the switches and the occupancy of the tracks . It does not know where the trains are going or whether they can go or not.
Lets assume that the train in the picture is Extra 1234 West. It holds an order that says “Extra 1234 West meet Extra 2345 East at Right Station and hold main track.” When X1234W stops at Right Station, the signals will look exactly as the picture. If the signals all the way to Left Station drop to red and the X2345E stops at Left Station, there will be no way the meet can happen. The X2345E has no way to get from the Left Station to the Right Station.
Actually the X2345W can leave Left Station even if the signals are red because it has a train meet at Right Station (which assumes no superior trains are due or overdue). It just has to proceed prepared to stop short of obstructions.
First, I appreciate your comments. I think i’m begining to understand what railway signals can and don’t do, that they provide protection, not authority.
I’m trying to understand what it would take to build an automated signalling system on a model railroad (i.e. no dispatcher). A signalling sytem that is not just fail-safe, but grants authority
After reading Railroad Operations and Railway Singnalling, I believe that signalling systems can determine direction. It would be fair to assume that if a trains entered a siding in a particular direction, it would leave in the same direction.
Not quite. In single track ABS they provide protection, not authority, however in CTC and Rule 251 double track current of traffic they do convey authority.
Just to be clear, you aren’t trying to devise an automated signal system, that is pretty easy, there are a dozen companies that will sell you components to do that. What you are trying to do is devise an auto mated dispatching system. That is WAAAAAAAAAY harder to do. The real railroads have spent decades and millions of dollars trying to do do that and none have a really solid solution. If you had a typical PC and it had a program to figure out how to best dispatch a major 20-30,000 mile class 1 railroad, it would take about 100 years for the computer to evaluate every alternative and come up with a SAFE solution. The problem is that to work the computer has to do that about once every 30-90 sec.
My suggestion is that you look at an automated sequential train operation. The system doesn’t really “dispatch” the railroad, that is figure out what is the best move, instead you load in a sequence of moves to be made and the system executes those moves sequentially, detecting when one move has been executed then authorizing the the next move in sequence. The caveat is you have to tell the computer what moves have to be made in pretty explicit detail and second you can’t deviate from the script.
[quote]
After reading Railroad Operations and Railway Singnalling, I believe that signalling systems can determine direction.
i’m trying to develop a better understanding of signalling and it limits. I’m thinking about a medium layout with a small number of trains running at the same time and the drivers of those trains switching turnouts so they can procede along their route.
My understanding of the term dispatch is that the dispatcher is determining the route and authority of trains. I’m not suggesting that a signalling system would route.
Beyond protection, I think a more sophisticated signalling system, using “overlap” and “direction detection” described in “Railroad Operation and Railway Signalling” could be useful to prevent two trains running in opposite directions from both entering the same multi-block single track.
Train drivers would route turnouts and the signalling system could provide authority. (Of course if there’s a single block between sidings, protection would be the same thing).
This probably isn’t prototypical, but ok on a model railroad (no one will get fired for running a stop signal).
I believe I understand you point regarding direction of trains assumptions when switching cars and validity of signals. So maybe a fascia switch to provide an input to the signal system to provide appropriate signalling for switching (i.e. what a dispatcher might do).
I lurk but don’t often post here. However, the subject you have raised is near and dear to my layout building heart and if I can help you out I would like that.
If you are in no hurry to get this system built, I don’t move fast [sn], then follow along on my LK&O blog. I am building a system that I believe is exactly what you are describing.
I am not attempting to replicate prototype signaling nor am I building an automated dispatching system. I simply want a working signal system that prevents collisions, and of course, looks cool. If I read correctly you want the same.
My layout is single track with sidings every 1.5-2 train lengths apart so there won’t be much in the way of single track lineside signals. The bulk of the signalling will be at each end of the sidings.
I fully realize the logic could be implemented with a micro-controller (Ex. Arduino) however, I enjoy classic digital circuit design so will use good ole NAND and NOR CMOS gates arranged in a truth table configuration.
Sound like fun? [:D]
So far I have all of the track related wiring and components installed - block occupany detection, turnout position detection, and direction sensing. All that is needed is the logic controller and the signals themselves. I am currently beginning fascia and control panel construction. Once those are complete it will be time to build the signal logic. A layout schematic can be found here.
What you are describing is a CTC system where the signal grants authority.
Just remember that is more or less a MANUAL system. A PERSON, the dispatcher, lines switches and then tells certain signals to display proceed. The Signal system does NOT determine which train goes where. The dispatcher does that and the signal system communicates the authority the dispatcher has indicated.
The part of the system you are trying to automate is the DISPATCHER.
In the ABS TT&TO operation, the signal system is just a safety overlay. The operation would be exactly the same with out without signals (as long as the trains have cabooses). The train orders or track warrants (once again issued by the dispatcher) determine the sequence of movements and the authority.
If you have train drivers, that would be Great Britain and they have a completely different system.
What you are talking about in this snip is having a sequential operation. You write down a list of things to happen, in what order, the train crews are given copies and they execute them in order:
X3604E
Switch Carpenter Steel.
Meet X861W at Cumru Jct take siding.
Set out/pick up at Birdsboro
Meet X6302W at Flat Rock
Yard train at Abrams
Then the signal system just reflects the movement of the trains. The sequence leads, the signals follow.
If I am a train at a siding on one side of a peninsula and you are a train at a siding on t
My layouts is not big enough for a signal system, but I’d like to understand signalling better. I know what a full blown system requires from the Pacific Southern I also have more than enough challenges at work.
The book I mentioned earlier (from 1942) describes implementing signalling logic with relays. As a TTL EE from the 80s, I find it amazing what they were able to achieve. But today you can pack oodles (? tech term) of logic into an 8-pin PIC processor.
Not to talk up the “opposition”, but the current series of RMC articles by Bruce Chubb on signalling may be very helpful to you.
IIRC he points out in one part that ABS just tells you if the mainline block ahead of you is occupied. It’s not tied in to turnouts or sidings. A train might come up to a green signal, and have to stop to throw a turnout to go into a passing track, because they have orders to meet an opposing train there. When they clear the main and throw the switch back to the main, the signal will turn to green again.
I’ve read Chubb’s part 6 where he describes Visual Basic code to control signals. It didn’t include turnout position.
I re-read Koester’s book on Design that included a section on Signalling. I thought he described the difference between ABS and APB as ABS would let trains enter the same stretch of single track but protect (fail-safe) them against both occupying the same block. APB uses “overlap” to prevent two trains from entering the same section of single track.
But I just received copies of RMC with Chubb’s parts 3 and 4 where he describes route and speed signalling.
It seems there are many types of signalling and grasping just some concepts is a challenge, much less the nuances between them … and then break the rules (and make assumptons) to make a semi-automatic system on a model railroad.
dispatcher controlled signals with interlocks, CTS, seems the easiest to grasp.
