# Distance between reversing curves



## CliffyJ (Apr 29, 2009)

Well, here I am once more, ignorant as all get out on track experience, but needing to lay concrete roadbed that doesn't guarantee derailments when I lay the track. So here goes.

It's my understanding (from reading / participating in the threads) that one needs 1 car length (I've been using 18" to be safe, in planning) between reversing curves.

Question #1: That just doesn't work in yards, with crossovers. How the **** does one create a crossover with an 18" min distance between curves, with tracks 8"-9" apart, without going to massively long (and $$$) switches? 

Prelim Answer #1: Yard speeds, being lower, are semi-exempt??? 


Question #2: Using flex track & rail bender on the main line, why would the trains derail anyway, even with zero tangency length? At full speed, yeah, they'd get thrown back and forth, and that would be asking for trouble. But how much of an issue is this? And would a 12" straight be sufficient? Or 6"? 

Prelim Answer #2a: There be voodoo here, and I'm not privvy yet to the private musings of the shaman... or the spankings of Mr. Beenthere...

Prelim Answer #2b: Please edumacate me...

Best regards all,
===Cliff

PS, you can probably tell that I'm having some real estate issues....


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## DKRickman (Mar 25, 2008)

Cliff, 
Here's my (admittedly limited) understanding of the issues involved: 

The problem with a reverse curve is that the couplers (if body mounted) swing away from the track center line, and long cars can have so much swing that they exceed the side travel of couplers, and one or both cars is pulled off the track. A car length of straight track between curves keeps at least one car in the straight track, thus reducing or eliminating the problem. Flex track (and the implied spiral easements) will definitely help, but the problem can still exist. 

Crossovers are not as much of a problem because the actual curvature is relatively small, and the entire car is never in the curve. The car never swings far enough to one side to cause a major issue. That said, short crossovers can be a problem for very long equipment such as passenger cars, auto racks, and the like. 

So no, I don't think there's any voodoo. It's all to do with the length of equipment and the coupler swing. 

Of course, truck mounted couplers on every piece of rolling stock also eliminate the problem (while causing others).


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## jake3404 (Dec 3, 2010)

I think Kenn hit the nail on the head on this question Cliff.


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## rdamurphy (Jan 3, 2008)

If you look at a prototype example of a crossover, you'll see that they only curve from the points to the frog, and then are straight from frog to frog - which is usually about two car lengths. We tend to have turnouts that curve all the way through the frog and are designed to replace a curve section in model railroading, not to mention impossibly sharp turnouts. The AMS #6's are build with straight track coming off the frog on the reverse leg. 

This might help: 

http://www.uprr.com/aboutup/operations/specs/attachments/amended/turnouts_std.pdf 

Robert


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## aceinspp (Jan 2, 2008)

In our hobby it's hard to be able to use the desired large turnouts to make the transitions that occur with our large passenger equipment. As said with body mount couplers and sharp curves its hard to avoid the cars from dragging one another off the track when operating through reveses curves unless you provide as much tangent track as possible. The more the better. Now if you have truck mounted couplers you can get by with less tangent track. Just be governed by the length of cars you will be operation and whether they have body mount couplers or truck mounted coupler. Later RJD


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## Greg Elmassian (Jan 3, 2008)

There are fundamental architectures in yard design that minimize S curves...

You might want to read my page on planning a yard: *http://www.elmassian.com...trong>**

You can see that there is a "yard ladder" where the turnouts all come "off" a straight line basically:








*


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## Dr Rivet (Jan 5, 2008)

Cliff 

Even on my layout which uses a c2c of 7.5 inches and #10 turnouts, the straight part of the crossover is only about 22 inches [including the straight parts of both frogs]. We can run full length passenger cars [ 30-34 inches] with body mounted couplers through at almost full track speed, including British/European models with working buffers. The point made by Robert is the thing that permits us get away with maybe a1/2 car length tangent is that the angle in the reversal is not large for #8 and larger. If you use a #6 with 8 inch track centers I think you will be OK especially if you aren't using cars like USAT full length passenger cars [33 inches approx]. Lay some temporary track out and run the cars through. Empirical testing is far better than theory. http://www.mylargescale.com/desktopmodules/activeforums/themes/MLS/save32.png


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## kormsen (Oct 27, 2009)

Posted By Greg Elmassian on 30 Aug 2011 08:51 PM 
* You can see that there is a "yard ladder" where the turnouts all come "off" a straight line basically:











yes, but... 
counting from the left you have a reverse curve between the second and third track, and in the back at the end of the yard you are stuck with a S-curve as well.

as much as i have planned and tried, a yard without S-curve seems to be impossible to realize.
*


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## Greg Elmassian (Jan 3, 2008)

The left hand track is the mainline, not continued in this picture. 

The right hand track in the foreground is a yard lead. 

The track in the far right is a second yard lead (this is a double-ended yard)... what you are calling an "S" curve in the distance has a long straight in the middle, so is very gentle, it is foreshortened by the distance from the camera, about 30 feet. 

So, there are no S curves within the yard, but the point is the design of the ladder minimizing the curves at both ends, by the geometry, the "straight track" that "feeds" the "ladder" of the body tracks. 

Regards, Greg


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## CliffyJ (Apr 29, 2009)

Thanks guys, you've given some great backstory to all this, and lots of additional points to consider. 

All my rolling stock (so far) has truck-mounted couplers, and is of shorter length (V&T RR). But I'll try to absorb and apply your advice, in case I get into more prototypical couplers. 

For the mainline, I've tried to be fairly strict about things, e.g., 18" between reverse curves; min radii of 5'; longer (for me) switches (Train Li's "R7") when the ML goes thru the arc, etc. 

However, I've been less stringent on spurs, where speed will be much less. Specifically, I'm using "R4" (4') switches and 4' min radii on spurs. And I think I'm pretty well stuck with that, in this first build. Should I just grab the Xacto and fill the bathtub now  or will the slower speeds resolve the issue (as I've been counting on all along...). 

Also, if a particular area of track seems too speed-sensitive, are there hard-wired speed limiting devices that can manage them? I think I've seen automatic end-of-track slow-down circuits, fwiw. 

All great stuff, thanks again. 

===Cliff 

Off-topic PS: Greg, is there a thread where you discuss the concrete board you use, pro's & con's, planting it permanently in the ground, etc.?


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## CliffyJ (Apr 29, 2009)

BTW, a lot of my issues stems from a rather anal-retentive desire to represent (in a compressed fashion, of course) the trackage of the V&T RR (Virginia City portion). Here's a shot of the (long term) yard plan:










The main entry switch is a 3-way (from Train Li), representing a rather famous one in the prototype. The middle (straight) leg is the main line. 

===Cliff


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## DKRickman (Mar 25, 2008)

Bear in mind that the reverse curves do not cause speed related problems, for the most part. Unless you're running so fast that the cars are being thrown outward significantly, the problem is strictly one of geometry. That said, I don't think you'll have a problem. 

On the subject of reverse curves in crossovers: 
The difference between a prototypical turnout and a toy-train switch has been mentioned. Using a proper turnout, no equipment will ever swing further than the frog angle, and that is usually not far enough to cause a problem since the next car is usually in a straight section. As mentioned, the track between the frogs should be straight, and that leaves a fairly long straight section between curves. I have seen very long cars (90' auto racks and passenger cars) run through very tight crossovers (#4, or worse) and it CAN cause a derailment - the cars are long enough to be swinging in opposite directions, and the crossover is too tight to accommodate them. Changing to a #6 crossover solves the problem. Other possible solutions include truck mounted couplers and couplers mounted on a swing arm to increase their side to side range. 

For what it's worth, we have the same problem on the 1:1l railroads. There are places where we are not allowed to handle equipment exceeding a certain length, or to have cars (or locomotives) of wildly different lengths coupled together, because the couplers will not swing far enough in tight curves and will cause a derailment. Just as on the models, one of the solutions is to use a longer coupler. The draft gear on every auto rack I've seen is incredibly long, and has several feet of side play. It causes the same sorts of problems that long couplers can cause on models, too.


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## jfrank (Jan 2, 2008)

It's a simple answer, the distance between reversing curves should be the length of your longest car. I would recommend 18" to 2'.


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## kormsen (Oct 27, 2009)

Posted By Greg Elmassian on 31 Aug 2011 10:36 AM 
The left hand track is the mainline, not continued in this picture. 

The right hand track in the foreground is a yard lead. 

The track in the far right is a second yard lead (this is a double-ended yard)... what you are calling an "S" curve in the distance has a long straight in the middle, so is very gentle, it is foreshortened by the distance from the camera, about 30 feet. 



sorry greg, i did not express exactly enough, what i wanted to point out.

when a train enters or leaves the second yard track beside the mainline to or from the front yard lead, it has to pass a left and a right hand turnout without any straight in between.

the same happens with the fifth yard track from the left and the yardlead in the background.
(these are, what i inprecisely refferred to as "S-curves")
i got the same problem at every station with a passing siding.
the two solutions i found, are 1) put the station building between tracks, thus gaining space for a straight, or 2) place the whole station (or yard) at an angle to the general direction of the main.

(as i won't even try to fiddle a pic into here, klick on the link:

sketched curves


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## Greg Elmassian (Jan 3, 2008)

Yep, the crossover from the main line to the first yard track does indeed involve an S curve, it may be hard to see in the picture, but those switches are Aristo #6.... the rest of the switches are Aristo WR (10' diameter) switches, about #4 frog. 

You get the train off the mainline and you can work it from this track. Once "there" your worst part will be working the first track to the right of that, since you curve right and then left. "Further out" in the body tracks, you have more straight track until you "turn left" to a body track. 40' cars have no problem, and neither do 50'. Longer cars want to be out "further". 

Best I could do in the confined space, but pretty close to a prototype design. 

Regards, Greg


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## CliffyJ (Apr 29, 2009)

Bear in mind that the reverse curves do not cause speed related problems, for the most part. Unless you're running so fast that the cars are being thrown outward significantly, the problem is strictly one of geometry. That said, I don't think you'll have a problem... 

Thanks Kenneth. That answers a big question.

Thanks all, for clarifying this stuff.

I guess my main problem will be re-planning certain crossovers in storage yards... that's kinda tough, because I was planning on a long shelf yard (under a long but narrow deck), four tracks wide, with crossovers that allow interchange in the middle (for storage of 8 short trains, vs. 4). I've seen so many LGB yards like that, I thought it was a no-brainer. But thanks guys, I'm sure you're saving me years of additional headaches.

===Cliff

[later clarification: if I used #6's, and gave the tracks better separation, it sounds like it might work. However, I'm using Train Li R4 (prior "R3") and R7 switches, which I think follow LGB geometry. Got a nice big stack of them. So to integrate code 332 turnouts, following "standard" frog / turnout geometry, in nickel plated brass (or whatever), from another vendor, might be problematic. Hence my inclination to just take the crossovers out of the plan. Not whining, just clarifiying.]


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## DKRickman (Mar 25, 2008)

Cliff, 

I would suggest building your yard just like you had planned. Unless the cost of the turnouts is an issue, they won't hurt to be there. If you do find that they give you problems with some long equipment, you can simply not use the crossovers for those cars. Given what you've said about your equipment at the moment, I don't think you have anything to worry about. 

Remember that every layout is designed for a maximum train length, car length, etc. There is little point in trying to make it handle equipment you are not likely to run. Better to accept a limitation and work within it. If you accept that you'll be running relatively short cars and locomotives, you can save yourself a lot of headaches and not worry so much about the reverse curves. Yes you prevent grossly different equipment from being run on your rails, but you gain the freedom to design and build what you want to suit the needs you KNOW you have today.


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## CliffyJ (Apr 29, 2009)

Kenneth, you know what, after hours of meetings today, yours are the wisest (and most encouraging) words I've heard all day. 

Yeah, in trying to model the V&T, I'm never going to have any engine bigger than a mogul, and no r'stock longer than, say, 14-16". I had to do a chop job on the only USA Trains cars I own, to bring them down to a more proto length. 

I especially like your "accept a limitation and work within it" rule. Very wise indeed. 

===Cliff


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## Ron Hill (Sep 25, 2008)

Cliff, you asked why "Using flex track & rail bender on the main line, why would the trains derail anyway" I can tell you that the least uneven track can cause a locomotive to derail in a curve. Aristro-craft six axle locomotives especially! I have very little trouble with USA four axle locomotives derailing on my mainline track which is all flex track because all the axles float on individual springs. But the A/C locomotives do not. The truck are a one piece mold and the end axle is stationary. I have found that a small uneven spot (a high point) just before a curve will cause the middle axle to pick up the front axle just enough for it to climb the rail as it enters a curve. I could not figure out what was causing this until I got down to the track level and watched it happen. Also, if you have a curve that is uneven in the opposite direction (twisted if you will) in the curve, the middle axle will climb the rail. In my opinoin, this is A/C biggest error on locomotives. If each axle had springs similiar to USA's four axle locomotives, the loco would ride the track a lot smoother no matter how uneven the track was. 
Ron


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## John J (Dec 29, 2007)

I have danced that dance before.

Like Ron Hill I got down on the ground and watched what happened when I first started my layout years ago. 

I watched troublesome curves. I watched troublesome switches. 


Hence the saying " Build it tuff or go home " ( I do like that motto Mr Lehrian ) 


I do concrete road bed and steel plates under the switches.

I know that concrete and steel isn't everybody's cup of tea.

But with Little or no maintenance you have more time to drink your tea and watch your trains run.


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## CliffyJ (Apr 29, 2009)

Well, I've been tweaking the plan, and the 1-car-long (for me, 18") min distance between reversing curves seems to work. 

A related issue. Let's say you're using a contant-radius turnout (e.g., Train Li R4 / LGB R3), and the track directly enters and exits with the same radius (the curved part of the switch is part of a circle). Not very prototypical, but bear with me. And let's say the straight leg of the switch is 1 car length. Upon that leg's exit, another curve directly begins, going the other way. 

Question: is there any reason why the switch's straight leg can't serve as the min distance between the described reversing curves? 

Inquiring minds want to know... 

Thanks again all, for your various tidbits of insight, 
===Cliff


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## CliffyJ (Apr 29, 2009)

Ugh, I just found that I'd overlooked a place with only 11" between rev. curves... and it's "cast in concrete." It's on a small spur which would usually only receive (short) ore cars. 

I think I'll just leave it be. If I have to later, there's room to pour a little more crete and grow that straight leg. 

Just thinking out loud, 
===Cliff


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## DKRickman (Mar 25, 2008)

Posted By CliffyJ on 03 Sep 2011 11:44 AM 
Question: is there any reason why the switch's straight leg can't serve as the min distance between the described reversing curves? 
Cliff,

A switch should be just another length of track to your cars. Assuming the switches are well made and maintained so that they are not themselves the cause of derailments, then they should (in theory) be no different than any other piece of track with the same shape.


Ken


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## CliffyJ (Apr 29, 2009)

Thanks for the confirmation, Ken. 

I thought so, but I've learned the hard way in this hobby not to take much for granted, ha ha! 

Hope you're having a nice holiday, 
===Cliff


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## du-bousquetaire (Feb 14, 2011)

I have found using continental equipment with buffers and screw and hook couplings that with N° 8 pointwork (or over) you don't have problems on crossovers. Aside from that, tangents between reverse curves should be the length of a full scale length passenger car or one of the modern 80' freight cars. I say this from experience. Running on exhibition layouts with shorter turnouts (Peco,or on the Swiss pointwork of the Twerenbold layout which are either N° 5 or N°6) cars will buffer lock in reverse systematically and may traverse safely if being pulled.On my former layout all crossovers were N° 8 and I never had derailments due to buffer locking. I have been running in gauge one since 1978.


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