# Handling Reversals



## icrr (Jan 21, 2015)

My layout is being built in sectons. Each secton has one or more modules. A module is 36" X 24". The first section I'm working on has three modules and contains a number of reversal points.

For this initial post I've attached a picture of the basic layout, "Reversal 1.jpg". At the botton of the layout is a staton area that will have an islsnd platform. Since this is a sngle track section there is a sidng onto which trans running clockwise around the section will be routed. The straght through track is for trains from other sections or those running counterclockwise around the section. And that creates this particular reversal problem. I know the station area and, perhaps some additional adjacent track needs to be isolated. Thus, gven my layoit is DCC, my questions for this problem are:

1. What needs to be isolated?
2. What do I need to accomplish a temporary reversal?
3. What do I need to do?
4. How do I sutomatically control the reversal and back to "normal" process?

Thanks. (There is another reversal problem involving the interior of the section that I will ask next.)


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## traction fan (Oct 5, 2014)

*Reverse loops*

ICRR;

Looking at your layout, I agree with you that it includes several reverse loops. Trying to cure all of them in one E-mail message would result in a very long, and confusing, message. Lets break it down into a manageable form. Starting with the loop on the left end; you would need to isolate a section of that loop's track, long enough to accommodate a train. This would require plastic, insulated,rail joiners on both rails,at both ends of this section. (FYI. this is often called an "X-section".)
You did not indicate what control system you plan to use; DC or DCC. 
As you model in Z-scale, I'm assuming straight DC from a power pack. Correct me if that's wrong.
The X-section is needed; and physically/electrically identical for either system, but automating the reversal of current is a bit different, and easier, when using DCC. 
The X-section will require it's own separate reversing switch. The "track" terminals of the power pack are connected to the "main line" bulk of your railroad, AND to the INPUT of the X-section's reversing SWITCH; but NOT directly to the X-section track. The OUTPUT of the X-section's reversing switch is connected to the X-section track.
In operation, a train moves along the main line with train speed and direction controlled by the power pack in normal fashion. When the train enters the X-section, that separate reversing switch must be set for the same direction of train travel as the power pack's reversing switch or the train will stop due to a short circuit, or oscillate back and forth because of the different direction settings of the main and X-section tracks. flipping the X-section's reverse switch will allow the train to keep going as it did on the main.
Once the train,(or at least the locomotive is completely within the X-section, flip the POWER PACK'S reversing switch. This will let the train exit the far end of the X-section, without stalling.
Wow! It takes a lot of verbiage to describe this. In the likely event that you're confused by all I've just said; write back, or check out a book titled "how to wire your model railroad". It's available from Kalmbach publishing Co. (www.kalmbach.com) or possibly your local public library. It shows what I just described in pictures and diagrams.
Each case where "reversals" occur, will require the same treatment. Systems exist, for automating all this,"throwing reverse switches" business, but are different for DCC.

Hope this helps;

Traction fan


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## traction fan (Oct 5, 2014)

*Reverse loops*

ICRR;

Looking at your layout, I agree with you that it includes several reverse loops. Trying to cure all of them in one E-mail message would result in a very long, and confusing, message. Lets break it down into a manageable form. Starting with the loop on the left end; you would need to isolate a section of that loop's track, long enough to accommodate a train. This would require plastic, insulated,rail joiners on both rails,at both ends of this section. (FYI. this is often called an "X-section".)
You did not indicate what control system you plan to use; DC or DCC. 
As you model in Z-scale, I'm assuming straight DC from a power pack. Correct me if that's incorrect. The X-section is needed and physically/electrically identical for either system, but automating the reversal of current is a bit different, and easier, when using DCC. 
The X-section will require it's own separate reversing switch. The "track" terminals of the power pack are connected to the "main line" bulk of your railroad, and to the INPUT of the X-section's reversing SWITCH; but not directly to the X-section track. Instead the OUTPUT of the X-section's reversing switch is connected to the X-section track.
In operation, a train moves along the main line with train speed and direction controlled by the power pack in normal fashion. When the train enters the X-section, that separate reversing switch must be set for the same direction of train travel as the power pack's reversing switch or the train will stop due to a short circuit, or oscillate back and forth because of the different direction settings of the main and X-section tracks. flipping the X-section's reverse switch will allow the train to keep going as it did on the main.
Once the train,(or at least the locomotive is completely within the X-section, flip the POWER PACK'S reversing switch. This will let the train exit the far end of the X-section, without stalling.
Wow! It takes a lot of verbiage to describe this. In the likely event that you're confused by all I've just said; write back, or check out a book titled "how to wire your model railroad". It's available from Kalmbach publishing Co. (www.kalmbach.com) or possibly your local public library. It shows what I just described in pictures and diagrams.
Each case where "reversals" occur, will require the same treatment. Systems exist, for automating all this,"throwing reverse switches" business, but are different for DCC.

Hope this helps;

Traction fan


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## Brakeman Jake (Mar 8, 2009)

According to the drawing shown,you have two legs of track that need to reverse polarity.They are the two upper legs of the X (in blue).Leg A will need double insulators at both turnouts 7210 and 7404 while leg B will also need insulators at turnouts 7403 and 7006.

Polarity reversing can be achieved with a reversing module (like Digitrax AR1).Depending on how many trains you plan on running,you may use the same AR1 for both legs.You may need a second one if you wish to have different trains entering the X section at the same time wich isn't much likely since they'd be on a collision course anyway.


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## icrr (Jan 21, 2015)

In my original post I broke down the problems as I see them. I asked for help first with the station area problem first. Solutions and advice that arr focussed on that issue will be appreciated. The other problems will be covered later. Thanks for your cooperation. BTW, the layout is being constructed for DCC from the ground up.


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## DonR (Oct 18, 2012)

You have a very complex set of Reverse loops.

But, first, to answer your question about the Station area. It is
a simple passing siding. Since you will be running DCC nothing
need be isolated on it. There would be no problem with a train
going clockwise on the main track and a 2nd train going
counter clockwise on the passing siding. That's part of the
reason why DCC is so desirable. And the car lights would stay on
while stopped.

Now, the real problem:

I see your layout a little different from Traction Fan and Jake, tho they are
correct in their suggestions.

I see a big oval with the 'X' within it. Now, the easy thing would be
to isolate the entire 'X' as a reverse loop. However. That would invite
problems. You will be running more than one loco, so the probability
of two entering that 'X' at the same time is great. That would result in
short circuits. So we can't do that.

It looks to me that you are going have 4 sections that will have to be
isolated, plus the middle section between 7204 and 7403 will need
to be powered by the main track. The reason is that you would not want
to have polarity reversing sections abutting each other. The 2 controllers
would get into a polarity fight, thus the isolated section using
the main power to separate them. 7204 through 7403 would be
main power.

You would use insulated joiners below 7210 and 7204.
Also from 7009 to 7204.

In addition they would be between 7403 and 7006
as well as 7403 and 7007.

This would require 4 DCC reverse loop controllers. There is 
a device available that has 4 controllers built into it. I will
look it up and post it.

The answers to all four of your listed questions are contained
in the above explanation. DCC reverse loop controllers operate
automatically. They are usually located under the table and need
no attention after installation is complete. The output of each
controller would power one of the 4 isolated reverse sections.
The main DCC buss would power the middle isolated section.

Don


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## Brakeman Jake (Mar 8, 2009)

Don...obviously we don't see this layout the same way.To me,the lower part of the X section is electrically the same as the lower leg of the oval,including both spur and siding,so that neither track within this part of the layout needs polarity reversing.

We agree that the crossing part of the X has to remain constant so that no two reversing legs don't touch.However,there seems to be something I don't see as I see no point in having the lower legs of the X reversing,other than adding complexity and costs.What am I missing here?


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## DonR (Oct 18, 2012)

Jake

You are right. I went back and looked at it again and I see your point
that the lower part of the X is simply a 'passing siding' for the main
oval. Thus, that reduces the isolated tracks to two, from 7210 to 7204
and from 7403 to 7006. 

So he would need only to use a Digitrax PM 42 which
has capability to control 4 reverse loops. It's cost
is less than buying two reverse loop controllers. Big improvement.

Don


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## icrr (Jan 21, 2015)

The interior of the layout has only two paths where reversals occur. These are shown in each of the attached files. Trains going across the upper portion of the interior and those going across the lower section of the interior do not involve a reversal. They exit the interior and return to the main line in the same direction as when they entered.

Don, you suggested the interior needs to be broken into a number of smaller separate (isolated) sections. My understanding is that an isolated section needs to be long enough to hold ALL of the longest train that will use the isolation area. When the layout is complete (not just what is shown in the reversal picture), some of the trains will be close to 4 feet long, which means the locos will have crossed at least one Y turnout before the last car clears the main line.

It is my plan to have only one train at a time passing through the interior to avoid conflicts and collisions.

It also seems to me that this entire section will need to be isolated from the rest of the layout because a train already in thus section may be traveling in a direction opposite the direction of travel of a train entering the section from other parts of the layout.


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## Brakeman Jake (Mar 8, 2009)

Most obviously,you have no understanding of how DCC works.First,DCC equipped locos will travel in the direction dictated by the digital instructions and WILL NOT be influenced by the track polarity,wichever it is.

The purpose of the polarity reversing modules is to avoid shorts when trains jump the gap between two tracks of different polarities by matching track A to track B or track B to track A depending on the specific situation.

The reversing modules simply protect the layout from trackwork shorts and have absolutely NO EFFECT on the direction of the train,wich you seem to think.As said earlier,the decoder equipped loco couldn't care about track polarity while the standard DC loco does.

As proposed,the lower section of your layout,including the spur,siding and the station area,doesn't have any polarity conflict,wich means that multiple trains can either follow eachother or meet freely as long as they don't enter the same turnout at the same time.


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## DonR (Oct 18, 2012)

Yes, I at first 'saw' the need for four isolated sections but Jake
pointed out the error of my ways.

The simple solution, as I have stated in my 3:15 post yesterday,
is to consider the entire lower part of the 'X' as mainline not
isolated.

You would then have only the two upper sections leaving off of this
as isolated 'reverse loops'.

You really don't need the isolated section to be longer than your
longest train. This would apply for lighted passenger cars or
caboose if the loco is still in the section.
The trucks of these would cause the controller to 
reverse. If the loco had already left the section that would
not matter anyway. About the only other resolution would
be to replace the X middle turnouts with a pure crossing.
That way each of the X crosses could be a long isolated
section.

The best laid plans...as has been said...you will get a 2nd train
coming through the X at some point, so you'll just have to be
on alert. With DCC, trains can be going in opposite directions
on the same track...just don't let them collide.

Do consider the PM 42 controller for your reverse sections.

Don


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## icrr (Jan 21, 2015)

Yes, I don't know very much about DCC, but I know enough about digtal systems to know that DCC doesn't care about the track polarity. That wasn't my concern. I assumed, perhaps incorrectly, that the loco motors were designed to accept electricity from one -- say the righthand -- rail and discharge it to the other -- say the lefthand -- rail. If this assumption were correct, the polarity of the track would matter. It would serm to me that a mismatch short occurs because the loco is expecting power from one rail and getting it from another (a simplistic way of putting it). My assumption about the design of the locos is based in part on the following quote from the Digitrax Zephyr Quick Start Guide:

"To run the locomotive, place the locomotive on your layout, move the Direction/Brake lever to either ‘FORWARD' or 'REVERSE', depending on which way you want the locomotive to move, and slowly turn the Throttle clockwise until the locomotive starts to move. If your locomotive runs in the opposite direction of the throttle setting, turn the locomotive around. To stop the locomotive, either turn the Throttle back to 'STOP,' or shift the Direction/Brake lever to 'BRAKE'."


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## icrr (Jan 21, 2015)

Don, thr UPPER part (from upper left to upper right and back) are NOT reversing. The ONLY reversals are those ndicated in "Reversal 2.jpg" and "Reversal 3.jpg". Those paths -- in either direction -- produce a reversal. The other paths (upper left-upper right and lower left-lower right) do not create reversals. It would, therefore, seem that whether the polarity needs to be changed depends on which leg is activated of the second Y that a train encounters. Does that change anything with respect to what is isolated?

I assume (I guess I'm making too many), I assume under DCC and using JMRI or sonethng similar that it would be possible to monitor and control what the trains do AND WHEN, being able to stop a train from entering the interior if another train is already in that area.

Mike


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## JerryH (Nov 18, 2012)

Yes, it's possible. It's called Train Controller by Railroad and Company. There are a very small number of modelers that go that far into computer control though.

http://www.freiwald.com/pages/traincontroller.htm

Here is a link to my RR using it. Look at post #313

http://www.modeltrainforum.com/showthread.php?t=14852&page=32


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## DonR (Oct 18, 2012)

Mike

I understand why you think the upper tracks where turnout 7210 and 7006
leave your mainline oval are not a part of the reversing. But they
are. Look at your layout this way:

It is one big oval with some parallel passing sidings and an X in the
middle. You can reverse the direction of your train running
clockwise by running
through turnout 7210, 7404, 7403 and back on the main at 7207 where
it will be now running counter clockwise..
You can also do it by leaving the track at 7009 going thru 7404 and 7403
and returning to the main at 7006. 

Now you can consider the section between 7210, thru 7404 and 7403 
to 7006 as simply a passing siding, likewise you could consider
the lower part from 7009 thru 7404 and 7403 to 7207 as a passing
siding. But both cannot be. A choice must be made which would be preferred. 
That would determine which two sections leaving 7404 and 7403 would need
be isolated.

Let me explain DCC: Your controller will put a modified AC current
of around 14 volts on your track. It will carry the digital information
from the controller to the addressed decoder in your loco. The loco
decoder will take the AC from the track, rectify it to DC, then raise or
lower the DC voltage it sends to the motor based on data from
the controller. It also reverses the polarity so the loco can go into reverse. 

You are right that current flows from one rail to the other. But on
an a DCC track the direction of that flow alternates back and forth,
which is why it is called Alternating Current. It does have what many
think of as polarity (phase). This comes into play when you have
the ability to turn a loco around to run the opposite direction on
the same track. You can see the problem if you take a red and a black
pencil and draw one rail in red and the other in black. Do this with
your main oval and also the tracks forming the X. You will see that
for example, when you leave 7207 the left rail is red as is the outside
rail of the oval. But going straight on thru to 7210 that red left rail
meets the black inside rail of the main oval. If connected without
insulated joiners there would be a short circuit.

The reverse loop controller solves this:

When the loco wheels span the insulated joiners the controller senses the
short circuit and lightning fast reverses the phase in the isolated track
so that is is matching the main oval. This happens so fast the loco
doesn't even blink and continues on. The reverse loop controller is
totally automatic and has only a sensitivity control that you adjust when
installing it.

So at this point you'll need to decide which sections to isolate. 

Don


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## icrr (Jan 21, 2015)

DonR said:


> Mike
> 
> I understand why you think the upper tracks where turnout 7210 and 7006
> leave your mainline oval are not a part of the reversing. But they
> ...


Don, I never said they weren't part of the reversng. I said, or at least meant, the top and bottom routes do not RESULT in a reversal of direction and polarity. A train following those routes leaves the interior going in the same direction as when it entered. ONLY those routes that go diagonally across the interior result in reversing th.e direction of travel



> Let me explain DCC: Your controller will put a modified AC current
> of around 14 volts on your track. It will carry the digital information
> from the controller to the addressed decoder in your loco. The loco
> decoder will take the AC from the track, rectify it to DC, then raise or
> ...


I appreciatr your taking the time to try to explain the workings of DCC. However, there are several errors which my knowledge and experience caught and which are confrmed by Kato and Digtrax. First, the current suppled to the track is DC and not AC. Second, in my experience (read my updated profile), all digital systens use DC (the AC goes through a transformer). The reason for this is an alternatng current inevtably flips the value of at least some of the bits in a data stream. Additionally, Kato in Japan -- in response to my inquiry -- said their locos are designed for direct current and accept power only on one side of the loco for proper functioning. Furthermore, the specs for my Digitrax Zephyr say the output is DC.


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## Brakeman Jake (Mar 8, 2009)

From what I learned through the years is that DCC is neither DC nor AC,though my understanding is that it works more like AC and also serves as a carrier for digital datas.What it exactly is I don't really know and don't really care to know either.What I know though is that it works like both DC and AC as it needs two paths (wires) that need not to be shorted together wich is the issue here.

There is a big issue here that is being overlooked that has been discussed in an other post...you cannot have two reversing tracks connected to eachother without expecting weird results.A loco jumping the gap between two un-matched reversals will instantly have both reverse module react thus an electrical dead end or simply put...a short.

To work properly,a reversing section has to connect to constant polarity (or phase) sections at BOTH ends.

With this in mind,there has to be a constant phase track somewhere in the middle,or where the reversing sections intersect,in this case the section between 7403 and 7404...no choice here.Then,you can decide wether the center section is going to match the upper or lower section of your layout.I suggested that the lower section be made "constant" (more tracks involved) but the upper would work just the same.

If you decide to match 7403-7404 with the lower part,then the whole section from 7009 through 7404,7403 and 7007 to 7207 becomes a passing siding that needs no reversing module.It also means that the spur connected to 7007 and the station area between 7008 and 7208 don't need reversers either.

From then on,the only sections that need reversers are 7210 to 7404 and 7006 to 7403 as I stated earlier.You could decide on the complicated (and expensive) way by having four reversers (either four AR1's or a PM 42) but you still need the constant phased mid section for this to work.Your layout is simple,you probably could get away with one AR1 for the two sections.


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## DonR (Oct 18, 2012)

Yes Jake

The track current is a square wave AC. Here it is explained:

http://www.sumidacrossing.org/ModelTrains/ModelTrainDCC/DCCDecoders/DCCVoltages/

All makes of DCC gear comply with NMRA standards so all output the
same currents with some voltage variations. 

Because it is AC you cannot permit a DC locomotive to sit idle on it.
The alternating + and - causes the motor to try to go clockwise, then
counter clockwise in a second, it can't, thus it heats and burns out
a winding.

We discussed that need for a neutral track to separate the isolated sections
a few posts back.

At this point, using the layout as drawn, and using a couple of the
arms of the X as the isolated sections the need for the 'neutral'
track is met by 7403 and 7404 being included in it. His problem
then is the possibility of a train too long for the isolated section.
Not a problem except for lighted passenger cars.

The only solution to that, as I see it, is to replace 7403 and 7404 with
an ordinary crossing. The entire 'X' would be split into two isolated
sections crossing each other. The isolated sections then would be
long enough.

Don


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## Brakeman Jake (Mar 8, 2009)

Very true indeed,a simple crossing is the solution.It wouldn't matter for a freight train but a regular passenger train could cross both gaps at the same time,lighted cars would be a problem.

I liked the first design as the X was in fact two passing sidings if desired...not possible with a crossing.


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## IlliniViking (Dec 13, 2009)

icrr, I see you're from Urbana. Not to derail this thread, just curious if you went to the show at Lincoln Square? I'm glad they brought it back, but it seemed a little small this year.


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## icrr (Jan 21, 2015)

IlliniViking said:


> icrr, I see you're from Urbana. Not to derail this thread, just curious if you went to the show at Lincoln Square? I'm glad they brought it back, but it seemed a little small this year.


Yes, I went to the show. Can't compare the size with earlier ones and didn't know it wasn't done for a while. I moved back here when I retired -- my wife was from here and I did my grad work here. So, I was away for quite awhile.


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## icrr (Jan 21, 2015)

I will be keeping the original design. It's purpose has always been to have the maximum variation possible within a small space. The whole layout is only 9' long (at this point). As long as crossing from one area to another doesn't destroy the car lights, I can live with them flickering. What will happen with them?


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## Brakeman Jake (Mar 8, 2009)

When a light equipped car jumps a phase gap,either...
1-no other loco or car is bridging the other gap,then the reverser will activate normally.
2-if other gap is bridged,then a short will trip your command station's breaker.


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## Brakeman Jake (Mar 8, 2009)

I wonder why it took me so long to think of it...what you need is a double-slip turnout (Peco SL390F or SLE390F) in place of your joining bridge in the middle of your X section.

First,a DST needs much less room so you'd have longer tracks on each side of the joint.Better still,you could use either legs of the X as reversing sections or regular sidings as needed.

This way,you still need only two reversing modules on one side of the X,either upper or lower.The double-slip turnout costs more but you'd save on two other TO's,so the extra cost isn't much considering the added operational possibilities.


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