reversing loops

[Anonymous]

can someone give me an a-z on how reversing loops work and how to make and wire them up. I am envisioning a single main line with a reversing loop on each end of the line, I do not, however, know the first thing about them. Questions, for instance, when a loco or a consist is in the loop how do I reverse the polarity and what will that do to any other working locos on the layout like a switcher or a loco on a passing siding? I know this is a lot to swallow so Thanks in advance. Mark

[selector]

A reversing loop does two things: first, it reverses the train. Secondly, it reverses the polarity of the track onto itself, and causes a short. To stop the short, you need to gap the loop in at least one place, and most of us would say to gap it at each of its extreme ends, just before the main line. If you make the loco cross the gaps without first reversing the polarity, the loco will bridge the gap and still cause a nasty short. So, insulating gaps, the train crosses into the loop, and exits to cause a short. If you reverse the polarity before it exits, there is no short.

Beyond that, I don't have a good handle on how the loco behaves when you reverse the polarity while it is in the loop. I have no such loop, so I have not learned more about it.

So, when you reverse the polarity, whether via a device or manually, since the loop is isolated anyway, other locomotives elsewhere are entirely unaffected.

[rolleiman]

Mark, Let's deal with one reversing loop and I think you'll understand how it affects everything else.. Let's say you have a single track terminating in a reverse loop, a track that reverses back onto itself via a turnout (WYEs work the same way btw).. Electrically, with 2 rail DC, when you do this, you'll have a dead short (not good) so as selector says, you have to isolate a section of track within the loop (both ends of the section).. It doesn't have to be real big but it does have to be long enough to handle your largest powered load (usually the locomotive(s))..

To wire this, you need a reversing switch to reverse the polarity of that reversing Section Before the train enters it.. Actually, you want to just make sure that the train will continue on it's forward path when it enters the reversing section of the loop.. Here's the tricky part.. While the train is on that section, you reverse the polarity of the mainline so the train can, again, travel on it's forward path..

Below is a diagram I drew for someone else here a few months ago..

[image]http://www.rolleiman.com/trains/blockwire.jpg[/image]

It is a control panel wiring diagram for DC cab control, 2 mainline cabs and a yard cab (3 total).. The one of interest to you here, is Cab C (lower right of the drawing).. Each of the boxes marked CAB are power packs (mrc, spectrum, etc). The other objects are Toggle switches, you are looking at the rear of them (terminals) in the drawing. The one labeled Turntable would be the one you would send to your Reversing section..

Now, What's going to happen to a loco operating on the mainline when you throw the power pack reverse switch? It's going to change direction (I'm assuming we aren't talking DCC here).. Your best bet would be to wire for cab control if you aren't using DCC and want to control more than one train at the same time.

There Are auto-reverse circuits that do this for you.. I think Circuitron makes one, Dallee may be another.. Or you can grow your own from something that can be found here..

http://home.cogeco.ca/~rpaisley4/CircuitIndex.html#index

Be aware that if we ARE talking DCC, you still need the reversing section.. The difference is, you can throw a reverse switch while the train is ON the reversing section.. Or, you can add an auto-reversing circuit such as the MRC (not favorable in my opinion), Digitrax PM42, I think Tony's Train Exchange also offers one as do several of the other DCC manufacturers.

Clear as mud, Right??

Good luck,
Jeff

[jwmurrayjr]

Mark,

Here's my contribution to the reversing loop thing. And this is a DCC application.

http://www.trainweb.org/silversanjuan/Page3/Page14.html

Merry Christmas,

[BR60103]

Mark:
For a basic operation you need 2 reversing switches (DPDT) that are outside the power pack. One controls the mainline and the two switches that define the reverse loops. The other controls the reverse loops, (could have an extra, one switch per reverse loop). Set the power pack reverse switch for forward, set the main line reverse switch for West. Run the train into the reverse loop, where the reverse switch is set for clockwise. After the train passes the turnout, change the turnout then change the mainline reverse loop to East. The train will then head back to the other loop.
When you get back to the first reverse loop, change the direction to CCW because that's the way you're going around the loop. (Or change the turnout position and go CW.)
If you have sidings to be switched along the way, you can use the power pack reverse for that.

[Texas Zepher]

Is this for just one train?

If so you can get by with just two DPDT reversing switches. One for the main and one for both loops. When the train gets totally into a loop - flip the direction on the main. When the train gets totally onto the main - align the next loop and flip the "loop" direction switch accordingly.

[Texas Zepher]

Is this for just one train?

If so you can get by with just two DPDT reversing switches. One for the main and one for both loops. When the train gets totally into a loop - flip the direction on the main. When the train gets totally onto the main - align the next loop and flip the "loop" direction switch accordingly.


Well that was strange a time delayed, stutter post. It told me there was an error and my message couldn't be posted. And in the mean time it looks like BR60103 has already said what I said.

[jecorbett]

Very simply, you need to have a reversing section that is completely isolated electrically from the main line. The reason is that when the loco re-enters the mainline, it is going to have the opposite polarity than when it entered the reversing seciton. So if the polarity of the reversing section is the same as the mainline when the train enters the loop, it will be different from the mainline when the train leaves the loop and when the loco bridges the gap, a short is created.

You need switches that will control the polarity of the main and reversing section(s) separately. The Altas Controller is designed to do just that with two direction switches, one for the main and the other for the reversing section. It is the simplest way to go.

You can work it one of two ways. The polarity of the two sections must match when the train enters the reversing section. When the train is in the reversing section you flip the direction switch for the main so that it can receive the loco coming out of the other end of the reversing loop. You do this with the Controller switch, not your power pack direction switch. That works fine for one train operation but if you have a second train running on the main, that train will change direction when you flip the Controller switch. If you are running a second train, the best way to do it is to stop the train after it has entered the reversing section. Flip the direction switch on the Controller for the reversing section and also flip the direction switch on the power pack that is controlling that train. Now the train in the reversing section will continue in the same direction it was going when it entered the reversing section and the reversing loop's polarity will match the mainline at the end the train exits the loop.

You can buy DPDT switches cheaper than an Atlas Controller but then you have to figure out how to wire them. The simplest way is to just spend a few bucks on the Controller.

[bill1224]

I'm working on a layout from a plan I got from "101 Track Plans for Model Railroaders" by Linn Westcot (#77, Jamaica & Kingsway ShortLine) before I knew what a reverse loop was/is. It has 17 required reverse switches, all clearly marked on the plan, but it's very difficult for me to see where all these loops are, as I'm sure most of them are of the "hidden" variety. Should I cross my fingers and assume the plan is correct and wire all those switches, or should I try to simplify the layout so that I don't have so many reverse loops? I'm afraid I could end up with a wiring nightmare if I venture into this unknown territory.
Thanx to any responders.
Bill

[Anonymous]

Just picked up N-scale after 15+ years since first playing around with it and had decided on Kato tracking system but am stumped as to reversing polarity. Does anyone out there know if there is a reverse loop "kit" for this track style? Any help would be appreciated!

[Anonymous]

mcr122, if you cannot find an automatic reversing system, you stand a chance of becoming unhappy with your railroad if you have to reverse track polarity by throwing a switch every time the train enters the reversing section. It got tedius to me, and my son, after a short while, and I ended up dismantling the layout and starting over with an easier track plan. Everyone is different, of course, so this may not bother you, but think about this first. Envision running trains and flipping switches before you do all that wiring. In our case, a slight distraction would lead to forgetting polarity, let alone track block assignments. You may want to make your track plan as easy as you can in DC wiring.

[fwright]

Bill - I count 4 reverse loops - 3 at each end of the "folded dogbone" and 1 turntable. Are you confusing gaps and/or blocks with reversing sections? My guess is the gaps are based either on blocks and reversing sections combined, or possibly the older selective routing turnouts. Don't have the text to know for sure. But I sure only see the 3 reversing loops. The tail of the wye combines with one of the reversing loops, so doesn't need a separate direction switch.

yours in reversing direction
Fred Wright

[jeffers_mz]

It's been said, but very simple this time around.

The reversing loop is going to reverse the train, right? Turn it around and run it the other way. You could cause that with the direction switch on the power pack, but then the loco would be running backwards. The loop will both turn the loco around AND run it the other direction. If you lift the loco off the mainline, turn it the other way and set it back down, without flipping any switches, it will point the other way, but when you dial up the trottle, it will run the same direction as before, but backwards now because you turned it around.

So how can a loop accompli***his? Well, it can't, not by itself. Say the plus rail is the one closest to you. Follow that piece of metal around the loop. Round around around....uh oh....the plus rail is connected to the minus rail now. Bad idea, that's a short circuit. Somewhere in the loop, the plus rail needs a break in it, to avoid connection with the minus rail. The minus rail needs a break in it too, for the opposite reason.

So what you do is, when the loco is exactly at the break, you flip the switch to reverse direction, and the loco continues perfectly on it's path around the loop, except....you aren't that good, nobody is.

So you put two breaks in both rails, one at the start of the loop, one at the end. Plus closest to you on the mainline, Plus closest to you on the first part of the loop, loop curves around and now the plus is on the far side from you, and while the train is on the loop, you flip a switch that only affects the mainline. When the train get's back to the mainline, moving in the opposite direction now, all is well and it keeps on moving.

The above layout consists of two electrical "blocks". But a reversing loop is not the only reason to wire up an isolated "block" of track. Maybe you have a yard where you'd like to run a switcher, independantly from the train making laps on the mainline. Maybe you want two different trains on the mainline at once. You could run two trains on the mainline, without blocks, but if one of the locomotives is faster than the other, and they always are, it will catch up and crash into the slow one, unless you have two blocks with two different throttles set at different speeds, or two blocks, where you can turn one off while the slow train makes up some breathing room.

Maybe you want to park a spare locomotive on a spur, and don't want it zooming off the table when you hammer down a different train on the mainline. All of these are reasons why you might want an electrically isolated block of track, but a reversing loop REQUIRES an isolated block of track.

Rule number two: If Lynn Westcott says to wire it up a certain way, either do what he says, or else understand [precisely why his way isn't as good as your way for YOUR SITUATION ONLY. Lynn invented model trains, way back when the John Bull was the fastest locomotive on any track anywhere.

You reverse polarity witha special switch. Two wires come into the switch from the power pack's variable terminals. Flip the switch, one way, and the power leavfes via one pair of exit terminals. Flip the switch the other way, and it leaves the switch through another pair of exit terminals. Two poles (wires), two possible settings (throws) this is a double pole, double throw switch, or DPDT. If the switch has a center position, no juice flows anywhere, this is a "center off DPDT" the kind you want for a parking spur for extra locomotives.

You bring the power pack wires into the switch's two center terminals, pick an end, and run two wires from there to the rails, one each. On the other end, you just wire up an X, so that the 2 terminals ion that end are connected diagonally to the two on the end with wires running to the track. Now power comes in, flows out the two wires to the rails, OR, power comes in, flows across the X, and then to the rails in opposite polarity than before.

What happens to everything else on the track when you do this? Going back to the loop, plus rail close to you, train enter's loop. you flip mainline switch to opposite polarity, right then and there, everything on the mainline comes to an abrupt halt, and immediately starts going full speed in the opposite direction, very messy. Unless you're onl;y running one train, because you waited till it was in the loop before flipping the switch for the mainline, right?

So here's the simple view:

1. You need one block (with it's own DPDT switch) for each reversing loop, and one for the the mainline.

2. You need one block with it's own DPDT switch for each loco that travelling in a different direction than another at the same time.

3. You need one block with its own DPDT center off switch for each loco you want parked on the layout while other locos run.

4. You need one block with its own separate throttle for each loco that's travelling at a different speed, regardless of direction, than another loco at the same time. If they are travelling at different speeds AND in different directions at the same time, each block needs it's own throttle AND it's own DPDT switch too. You can make this happen with two throttles using the reverse switcvh on the throttle, but trust me, don't.

The only way to keep track of what's happening on your layout, electrically, is to have a diagram of the layout on the control panel, with the DPDT switches located on the diagram IN THE BLOCKS THEY CONTROL, and wires up such that when you point the switch to the left, the loco moves to the left. If you point the switch right and the loco moves left, or if you point the switch up and the loco moves down, reverse the wires leading from the switch to the track.

[bill1224]

fwright & jeffers_mz,
Many thanks for your responses. I assume one or both of you has the 101 Track Plans book. pg. 2 in the front of the book has a little legend that says "letter R indicates separate reversing switch is needed" and a big black bar indicates an insulated gap across both rails. I counted 17 separate R's on plan # 77 so I assumed that meant somewhere on the plan there were 17 reverse loops. That seems like an awful lot of DPDT switches and it would take an expert to operate the layout, so I could be wrong on my assumption. (hope so). Westcott I guess has passed away and I assume this layout design is pretty old. The original publication date for 101 Plans is 1956. It will take me a while to digest what jeffers_mz says in his (your) response, but it sure looks good. I'm going to print it for futuure reference, so THANK YOU. Fred Wright, it would be real helpful if you could indicate where you think the reverse loops are. This is probably not the place to carry on an extended conversation about my special problem so if you think it's appropriate, you could e mail me at [e-mail]. (Hope I don't start getting SPAM from others). Thanks again.
Bill

[jeffers_mz]

Just because there's an "R" on the layout doesn't mean there is a reversing loop.

Think of a single spur. Train backs in, uncouples some cars, goes to pull out onto the mainline again. If you don't have a reversing (DPDT) switch, then the only way make this happen is using the direction switch on your powerpack. Do that while another train is running on the mainline, and it stops instantly, starts trying to run full speed in the opposite direction, and you have an accordion style train wreck.

If you have a switch that only controls that spur though, you can move back and forth on the spur while another train continues round and round the mainline, by flipping the spur switch while the power pack switch stays put.

I have 6 blocks and 6 DPDT switches on my 4 by 8 layout right now. I just finished electrically isolating all the spurs, and will add another 7 DPDT switches tomorrow, for a total of 13. It may seem like a lot, but with the track diagram on the control panel, and each DPDT switch mounted in the spot on the diagram corresponding to the section of track it actually controls, it's not hard to keep track of which switch does what at all.

[bill1224]

OK, my thanks again to jeffers_mz. I'm slowly getting the picture.
Bill