Double Track Mainline- What Diameter Curves To Prevent Overhang Interference?

Increasing the centerline distance and the curve radii are the two variables. Of course, smaller steamers is the third variable. You'll have to play with those. When you run the beasts, you need lots of space. Even with huge curves, there is often overhang with the large articulateds.

I use multiple diameters of Ross sectional track for my double-track mainline with curves ranging from O72 to O80 to O120 to O128 on my mainlines; obviously the overhang is less as the curve diameters increase, and in general I tried to maintain a 4.5" center to center rail spacing. I can tell you that is cutting it close on the lesser diameter curves, but thus far no collisions I don't have the really long articulated engines in my roster so it's probably best for you to experiment and allow for a margin of error.

I tried to maintain at least 5 1/2" spacing, all my curves are O72 to O90.  The most overhang I've ever checked is the MTH Premier Big Boy with a 3.25" from track center line overhang on O72 curves.  That engine cleared the MTH autorack on the outside curve with the BB on the inside.  Not by much, but it did make it by.

Paul & GRJ,

Thanks for your real-world feedback on my question of curve clearance. I guess I will re-check my C&O Allegheny on my existing 72” diameter outside curves again to see what the over-hang measures. GRJ- your experience with your BigBoy on getting 3-1/2” overhang on 72” diameter curves is great to hear. Maybe I just haven’t paid enough attention to realigning my existing curves which are not screwed down. The existing 72” & 63” Gargraves curves may also be causing issues as the double mainline has a few transitions from “inside” to “outside” curves. Based on measuring the Allegheny on those curves again, maybe upgrading to the 80” & 72” diameter curves and get rid of those 63” curves might still work.

Carl J

If you make the curves non-concentric you buy yourself more room. Use the same radius on both tracks, but start the inner curve 4.5" (your centerline spacing distance) before the outside track. You'll end up with it being about 5.5-6" apart at the midpoint of a 90 degree turn, which is plenty of room to clear. We built modular layout corners like this and tested them with C&O Turbines (the king of overhangs) and scale auto racks... it all clears.  

On less than 90 degree turns you can do the same thing, sometimes by inverting the radii (using the larger radius on the inside) and continuing the straight section of the outside track for a bit longer before turning it.  Some experimentation is necessary, as sometimes you have to skip a radius in between (i.e. use a 96 and and a 72, skipping the 84).

If I'm reading the NMRRA standards right, 5 1/2" center-center should be safe for modern locos and rolling stock on radii down to ~64". Like others have said, you may be able to get a little closer with wider radius curves.

Al, Paul, GRJ, Boiler Maker, Mike

Everyone,

Thanks for the additional comments on the curve radius. Boiler Maker- your suggestions on looking at starting the inner curves before the outside curve & using multiple radius curves was interesting. I had read about this technique but I could not picture how to use different radius curves to get the “curve output” to be the consistent center-to -Center that I started with at the beginning of the curve. Using different radius curves I guess makes up for the potential changes in center-to-center spacing?

I guess i did not realize that I have always liked the look of fully concentric curves ( I did not even know there was this name description)  but now I think I will look at that non-concentric option as well.

Thanks to all!!

Carl J

I don't have any of those huge steam locomotives, but on my last layout I was concerned about clearance, too.  I had a 6 X 15' layout with an O-64 outer loop and O-54 inner loop using Ross switches and sectional track.  I put a 3" long track section between the switches from the inner and outer loops.  My largest locomotives are both from MTH, an FP-45 and an ES44AC.  Neither one sideswiped a train on the other track with that approach to clearance on curves.

Here's a look at the same area from the other side of the layout.  If you need a point of reference, the tank car with the blue ends in the center of the photo ago is in the left upper part of the photo below.

Maybe it wasn't elegant approach to clearance on curve, but it worked in this case.

I hope this info is helpful.

5.5" center to center is the min you want.  Anything closer and you'll hit.  Lionel FasTrack is set at 6" centers and works perfect.  Since my layout is Atlas track, and too small to run large stuff on the inside, mine is like 5" centers (Maybe even 4.75" at one spot.) and it works fine with the smaller rolling stock.

FYI

The Lionel LCCA FasTrack modular kits are set up for the use of O-84 curves on the outer loop & O-72 curves on the inner loop.  These are 6” apart center to center.  

@CBQ_Bill posted:

FYI

The Lionel LCCA FasTrack modular kits are set up for the use of O-84 curves on the outer loop & O-72 curves on the inner loop.  These are 12" apart center to center.  

That is 12” difference in diameter, and 6” difference in radius. That makes the track center to center 6”.

5 out of 4 people have trouble with math

I agree with using fastrack diameters as a reference. Seems like the 6 inch centers keeps you out of trouble.  

Optimum track centers depend on the amount of curvature and the era and type of equipment you run.  The NMRA publishes guidelines for this.  The FasTrack and Gargraves standards of 6" are definitely safe bets, but if you have wider curves you can get away with less.  The absolute minimum on straightaways is 3" for the 19th century era and 3 1/2" for the modern era.

https://www.nmra.org/sites/def...enters_july_2017.pdf

To put it in perspective, the Horseshoe Curve is probably one of the sharper curves on a class 1 mainline.  It's about 9 degrees, which in 1:48 works out to a radius of about 150" or 12.5 feet.  At that radius, the NMRA recommendation for track centers would be just shy of 4 inches. 

The BEST answer is as far apart as you can tolerate.  the length of engines and cars and overhang from the past to today has changed.

So the distance for ALL if them is difficult to say. If it's just the engines and cars you have today; then take the largest ones and with a pencil or sharpie held to the car's mid-section move the car and scribe a line on the layout where adjacent tracks are close; do the same scribing at the end of the car. AND Do the same on the adjacent track(s). The scribed lines shows you where NOT to put track or trackside accessories.

For reference here is the mth premier big boy on O72 for my inner circle and an mth heavyweight (18”) on the outside circle at just about O82.  As you can see, they barley touch and need another 1/2” of clearance to be able to operate like that.  

At the time of layout construction, I didn’t own or want a UP Big Boy, and was using my DMIR Yellowstone (2-8-8-4) for a clearance reference.  I planned to run only the smaller 18” passenger cars (no scale auto racks or 21” passenger cars).  Things worked good, but then UP made plans to restore the 4014.  I went to see it in April of 2014 (picture below) just outside of Barstow CA as it was drug from CA to WY in transport for restore.   The UP big boy restore has really been a lot of fun to follow along with and see it when it visited KC a couple of years ago and did really change my mind in wanting to own one.  I am glad I have it now, it just can’t run on the on the inner loop if passenger cars are on the outer loop.

If your alright with the smaller 18” passenger cars and the big boy is important to you.  I agree with the 5.5” centers, other wise look bigger.  If you had another steam engine you were think of that was smaller than the  big boy, let me know and I might own it and be able to post the same picture reference with the different engine and the 18” passenger car.

Thanks!

Mike

When checking the overhang (especially on articulated locomotives), don't forget to check how far the cab swings out.

The layout building book which contains a series of OGR articles by Jim Barrett includes a chapter on a real world non-mathematical method for marking clearances on curved track.  He measured his maximum overhangs, cut a stick to the necessary dimension (outside and inside might be different), and attached it to a truck that is rolled along the track while using a pencil to mark the location. 

@Hump Yard Mike posted:

For reference here is the mth premier big boy on O72 for my inner circle and an mth heavyweight (18”) on the outside circle at just about O82.  As you can see, they barley touch and need another 1/2” of clearance to be able to operate like that.  

If your alright with the smaller 18” passenger cars and the big boy is important to you.  I agree with the 5.5” centers, other wise look bigger.  If you had another steam engine you were think of that was smaller than the  big boy, let me know and I might own it and be able to post the same picture reference with the different engine and the 18” passenger car.

Your photo illustrates the problem one is trying to avoid perfectly.  I will say that I've found the articulated locos tend to have a 1/2" or more of play in their left to right movement.  Using a small spring to pull the front of the boiler can help clearance issues, though you have to make sure it doesn't push the cab out too far.

Pat, Sinclair, CBQ Bill, Ron, Rod M, Rplst8, Alan, Mike, Mallard 4468

Thanks for everyone’s input on my question for track diameters to avoid interference on the curves. The pictures and comments are really helpful.

Thanks so much!

Carl J