Figure of 8 - reverse loop module needed?

[Ralphmp]

I've been advised that if you have a powered layout with a cross-over (e.g. a LGB 13000 or a LGB 12260 double slip) in it that creates a "figure of 8" you don't need a reversing loop module. Is this correct, and if it is why isn't there an issue?

I've drawn this out several times and I always end up with a polarity swap between the different "loops" so what am I not understanding?

Any help in simple terms would help un-addle my tired old brain!

Many thanks
Phil

 

[playmofire]

Certainly in the case of the figure of eight with the 13000 crossover, there's no problem because the crossover itself is isolated so there is a break in the current. This has the advantage that there's no polarity swap but, for short locos, the disadvantage that the loco may "stutter" or even stall as it crosses the crossover.

 

[PaulRhB]

If it's a slip or point you'd need two modules one for each side unless you can have a big enough centre section that the train can't bridge both loops at once. Someone did it with one and metal wheels bridging the two loops gaps caused problems as they'd only used one module.
I guess whoever told you that it's not going to need a module assumed you were using the 90° crossing not points or slips. The 90° crossing is isolated so you don't get a short as the rails don't meet.

 

[Zerogee]

The "figure 8" with a simple crossover is one of the most basic "train set" track plans, and wouldn't be anything like as popular if it required any fiddling about with polarities! Any basic commercially-made crossover track (30 degree, 45 degree, 90 degree or whatever) will not cause a polarity problem - as others have said above, the rails are isolated at the actual crossing so that no short occurs. A double slip is a different matter, I think, though I'm not 100% sure of that without tracing all the electrical connections in one of them.
So the quick answer is that with a simple crossover to make a figure 8, you don't need to worry about polarity at all.

Jon.

 

[Ralphmp]

It all sounds fine but I still don't understand how with a double-motor loco - e.g. my Mallet - how it copes when crossing over as it has with one motor with the +ve rail on one side and the other motor having it on the other side (if you get what I mean)... Sorry if I'm being thick here

 

[trammayo]

If it is a plain 'figure of eight' with a straightforward crossover (22, 30, or 90 degree) the polarity does not change BUT, if you use a double slip there will be a polarity problem.

 

[playmofire]

Starting at one end of the crossover, put your finger on one side of the rail and run it slowly along towards the other end of the crossover. You will find that after a certain point the brass rail ends and there is a brown plastic piece, which also ends and is followed by another piece of brass rail. The rails running through the crossover are not continuous - they are broken - and so the electrical circuit is also broken. And that is why there is no reversal of the polarity (and also why small locos such as the Stainz and the Spremberger and the Playmbobil 0-4-0 tsnk can stutter or even stall). Your Mallet, because it has two motors won't stutter or stall because there is always one motor either side of the gap in the crossover.

 

[ntpntpntp]

Phil [ralphmp] perhaps you need to confirm with us whether your figure 8 design uses a simple crossover or something more complex such as a double slip or back-to-back points? As has been stated, we wouldn't expect a short circuit if it's just a simple crossover.

 

[Zerogee]

Phil - to clarify it in your mind, just think of a simple crossing in a figure of 8 track as being exactly the same as if you made a figure 8 with a bridge so that one track physically crossed above the other - you wouldn't have any electrical problems there, obviously....now, all that the crossing piece does is to squash that down into two dimensions, by electrically isolating the rails from each other where they cross by means of plastic inserts similar to those used for point frogs - the polarity for the loco remains the same as it runs over the crossing, just as it would running over a bridge! At no point do the actual rails touch each other electrically, so no short-circuits - the electrical continuity of the track is maintained by wires or bus-bars under the sleeper webs, which again cross over each other while being electrically isolated.

That will either make it crystal-clear for you, or even more muddled....

Jon.

 

[Ralphmp]

THUD!!! Yep, penny has dropped at last! Your collective patience is much appreciated. I think I confused myself when I was trying to work out where I will definitely have polarity issues and where I won`t in the schematic below. This is a much-simplified view of my layout which at present is basically an oval within an oval. The extensions I`m working on will change this to first introduce a figure of 8 using the straightforward crossing (this is an extension of the inner oval). Thanks to your help I now see that using the straightforward crossing won`t introduce a polarity issue.

As for the extension to the outer oval, I know the loop at the top right will require a reversing loop module, but presumably that`s the only place I need one? Even though I`m using a double-slip switch, if I`ve understood things correctly, I have only created a kind of figure 8, not another reversing loop. Is this right?

(I realise it would have been far easier if I`d posted the schematic initially - sorry!)

 

[HBBahn]

The loop to the right and the loop to the left are both return loops, but if the section of track between the point and the double slip if long enough, I believe you can get away with one reverse loop module on that section only. That section needs to be long enough to hold your longest loco.

 

[Ralphmp]

Thanks Tony

If things go as planned, both the distance from the double-slip to the reverse loop point and the length of the reverse loop will both be large enough to cater for the longest trains I run. (Not sure if it make a difference but I use Massoth DCC throughout the layout rather than analogue.)

 

[HBBahn]

Ah, just to be clear, I`m talking about the section of track between the double slip and the point to the left of it in your plan, not the point to the right associated with the obvious return loop above the main layout.

 

[Zerogee]

The "inner" dumb-bell track which is completely separate from the outer line looks to have no problems at all, using a simple crossing there should be fine. However with the "outer" line and the spur out to the return loop, you've actually created THREE return loops as far as I can see, due to the presence of the double-slip. Apart from the obvious one at the end of the spur, you've made both the left (main) part of the outer loop AND the right hand end of it into return loops.
Replacing the double slip with another simple crossing track, though this would cut your operating freedom down a little, would remove both of those loops.

Jon.

 

[PhilP]

The "inner" dumb-bell track which is completely separate from the outer line looks to have no problems at all, using a simple crossing there should be fine. However with the "outer" line and the spur out to the return loop, you've actually created THREE return loops as far as I can see, due to the presence of the double-slip. Apart from the obvious one at the end of the spur, you've made both the left (main) part of the outer loop AND the right hand end of it into return loops.
Replacing the double slip with another simple crossing track, though this would cut your operating freedom down a little, would remove both of those loops.

Jon.

Wot Jon said..
Had looked at it last night, but was too tired to work it out..

 

[Ralphmp]

Right. Thanks. Back to the drawing board but somewhat wiser on the implications of using cross-overs and double-slip switches.

 

[dunnyrail]

Right. Thanks. Back to the drawing board but somewhat wiser on the implications of using cross-overs and double-slip switches.

Glad you are having a rethink, have to say I was having trouble trying to understand what you were trying to create with that Track Plan. Think you might find the Railway somewhat better if you could at least have some way of getting Trains from one circle to the other. Though you could use the crossover to have 2 equivalent circles in a big figure of 8 without drastic changes to what you have drawn. Then either ditch the reverse loop or have one inside the Circle if you land will take it.
JonD

 

[PaulRhB]

The loop to the right and the loop to the left are both return loops, but if the section of track between the point and the double slip if long enough, I believe you can get away with one reverse loop module on that section only. That section needs to be long enough to hold your longest loco.

But be aware of stock with metal wheels or metal wheels plus lighting as they can bridge the gap the same time as a loco on the other one and cause problems just something to consider if you have weird behaviour and simply fixed by a second module if you have such stock.

 

[Ralphmp]

Just wanted to clarify that the schematic I posted was a simplified diagram to help me understand the figure of 8 issues.

My actual trackplan is shown below. You`ll see that links do exist between the inner and outer loops, and also (hopefully) why I want the additional reversing loop - it`s to allow me to extend around the edge of the garden whilst retaining my current continuous loop capability. (NB - the figure of 8 extension to the inner loop is not shown on here.)

I`m now looking into the possibility of making the extension double-track and avoiding a double-slip completely which should remove any reversing loop issues. I`ll keep you posted.

 

[dunnyrail]

Aha the plot bocomes clear, sure that I have seen this plan before, well without your extensions.

If I were you I think that I would forget the bit with the X crossover in your existing Track Plan on the left. Use that instead of the Slip to give your Double Track Loop as in this suggestion. No Loops No revercing Modules required.



As for the Slip, well you could think about incorporating it somewhere in your Station. they are a great space saver.
JonD