Track Spacing - center to center

NMRA TRACK STANDARDS are here. You can't go wrong adhering to those standars.

All the curve radii are expressed as RADIUS on this page. Don't forget to double the radius to get the diameter. For example, a 72" RADIUS is an O-144 curve. 3-rail O is the only segment of the model railroad hobby to use diameter instead of radius.

Thanks. I used this site previously when I dabbled in HO.

Never crossed my mind to look for O info here.

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You can't go wrong adhering to those standars.

Unfortunately you can. At least most 3 railers can.

If you check out the NMRA standards and you read the definition of Class Ia...

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Class Ia
Includes the largest steam locomotives with four-wheel trailing trucks, articulated locomotives, those with rigid wheelbases in excess of 20 feet, full length passenger cars and other long rolling stock.

... and then look at the chart you are told to "See note 8" for any radius under 58 inches. That's O-116!

Note 8 states:

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Construction or operation of equipment on curves sharper than those listed for its class are neither prohibited nor recommended.

So the NMRA recommends against operating a Berkshire, Northern or Big Boy on every three rail layout out there!!!!

Decades ago when scale Hudsons and Madison passenger cars defined big three rail O equipment 4 or 4 1/2 inch track centers were fine. Now that we have several brands of scale passenger cars and scale Challengers, Yellowstones and Big Boys running on O-72 curves that spacing isn't broad enough.

Six inch centers on curves will provide adequate clearance for an articulated or electric to safely pass on an inside track while scale passenger cars travel by on the outside track. That means O-72 inside of O-84. You can save a fraction of an inch as you get larger than O-84 but only a fraction until you get into the range of the very broad curves on the NMRA chart.

Prior to posting this question I was planning on having around 4" - 4 1/2" track centers anywhere pieces ran close to each other.

It sounds as if there is no clear standard since products are evolving and may continue to do so. The best answer depends on what type of items one plans to run. I don't want to settle on something and find out down the road that I wish I picked a different spacing.

Is there a number that others have settled on or how about I don't have any tracks that are closer than _________ " rail centers?

Thanks
Matt

I can verify Dr. Hikel's comments.
If a Big Boy passes a 21" coach on a curve you learn all about track centers.
Make them no less than 5.5" or, preferably, 6" as the Hikel Rule states.

Eliot

This is what worked for me:

I have a double-track line with 054 curves and 072 easements, using old-style conventional track with 16 pieces to a circle for 054 and 072. I used a minimum center-to-center track spacing of 4.5" on these mostly 90-degree curves. This clears an MTH big-boy and "baby Madison" passenger cars on adjacent tracks. The MTH big-boy is not scale; the length is compressed. My semi-scale passenger cars are only 13" long.

You will need to compromise between closer track spacing for better appearance, and wider spacing on curves for clearance of the largest equipment that you intend to run. I really didn't plan to have a big-boy, and I had to widen my track centers on curves for it.

Of course, wider curves for long equipment are better, if you have space for it.

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Is there a number that others have settled on or how about I don't have any tracks that are closer than _________ " rail centers?

The number with which to fill in the blank is 6 inches.

If you have curves of O-72, O-84, O-96 and O-108 you will be able to run trains with the largest scale locomotives and scale length passenger cars or 89 foot flat cars without problems.

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I can verify Dr. Hikel's comments.

Eliot

Thank you very much for the honoris causa degree. And thank you for coining the phrase "the Hikel rule." Dave got a kick out of that.

Since I got a Challlenger in 1993 I have used 6" on center spacing between tracks and from walls/edges.

On 072 a Challenger's outreach at both the rear cab roof and at the front running board corner is 2-1/8" as measured from the center rail. The outreach at the corners is virtually the same for an 80'[20"] passenger car[2-1/8"]. On both the engine and the railcar the overhang at the center on the inside of an 072 curve is 1-3/16". As regards the latter dimension, for engines it depends on what is hanging on the side of the various articulateds and 8-10 drivered engines--FWH, Air Pumps, etc.

I got my measurments by placing the engine/cars in the curve arc and using a small square against the outreaching protrusion[cab roof,etc], marked the benchwork and then measured back to the center rail. My outer track where I measured is 084 and I have tried the Challenger and a Y6-b opposite one another on the 072/084 curve without a problem.

Be aware that the inside overhang on big engines/long cars is enough that it will trash a Switchstand Lantern when negotiating curved switches unless you extend the ties out where mounted.

Hi all,

The current champions for swing out on a curve are the MTH DM&IR Yellowstone and MTH UP Coal Turbine. Both swing out 3.5 inches from the center rail on O-72 curves.

The current champions for encroachment on the inside of a curve are Weaver 21 inch passenger cars and the Atlas 89ft. flat car. Both encroach by 2.375 inches from the center rail on O-72 curves.

The decrease in these measurements between O-72 and O-84 is negligible. Add 3.5 to 2.375, and you get 5.875 (5 7/8) inches. That makes 6 inches your fool proof spacing.

It also happens that a six inch spacing works well in tighter radii with tinplate and sub-scale equipment. It's no accident that Lionel FasTrack comes in 12 inch diameter (6 inch radius) increments. Most of the NMRA standards and RP's for Hi-Rail O track work are decades old. Many of them are simply out of date with modern equipment and many were never valid to begin with. The figures they give for check gauge, flange way width, and several other measurements in a switch are totally incompatible with production O-gauge cars both post-war and modern. The NMRA doesn't pay much attention to 3-rail O and people in 3-rail O shouldn't pay much attention to the NMRA.

Thank you very much. I can make 6" spacing work. It does not look bad either. I cut out numerous sizes of curves from cardboard and having been playing around with them. I marked the center rail on them all.

On straight track don't forget the spacing on bridges. Bridges Unlimted used four inch centers. I don't know what atlas uses?
Clem

The Alas track system maintains a 4.5-inch center rail-to-center rail distance "everywhere".

 

Jan

I hope this note helps in this discussion.

As some have mentioned above articulated locomotives and long passenger cars are the problems when dealing with curves.  Standard distances between tracks are not the rule here. In three areas on my layout I have three track running toghther in a 72 in. curve. 

When laying the track I got out the AC-9 and some 21 in. passenger cars and ran them back and forth in the curved area on adjacent track.  For these items to pass track center rails vary in distance from 5.5 to 7.2 inches.

On most of the straight of ways I can get the center rails to be about 4.5 inches.

I hope this information is of some value.  Using the offending locomotives and passenger cars by testing when laying track will keep you out of trouble later!

 

Mike

 

Two switches set up for a crossover will also drive track centers.

If you're using flex track, you can vary the spacing to suit your needs. On tangent (straight) track, you can run the track as close as 13 feet (3.25 inches) and spread the tracks apart going into curves. You can use a larger easement to go into a 36" radius (O-72) curve on the inside and a smaller easement to go into a 42" radius (O-84) curve on the outside. That gives you safe spacing on the curves while maintaining closer spacing for straight track. The other alternative is to make curves so broad that the distance in-between doesn't matter much. We have a stretch on the layout that has 4.5" spacing, but even the biggest articulated locomotives keep their boilers over the track. The curve in the foreground is the sharpest in this stretch at about 96" radius (O-192) and the lead-in curve at the far end (extreme left) is so broad we couldn't estimate it accurately.

 

Here's a shot from the other end. The track on the other side of the bridge is at 48" radius (O-96) for the inside curve and 53" radius (O-106) on the outside, being spread apart via easements. We've had full scale equipment running through that curve for years and nothing has ever hit.

 I went through all this last year. If you want to be able to run anything out there use 6 inch minimum.   

A new Layout Planning and Construction section with
Track Clearances for Curves Tunnels, and Pantograph Skids
Accessory Footprints - Postwar and Modern
has been added to my website.

Correct me if i'm wrong but, I think the diameter of the curves would dictate the spacing, for example: you would require less spacing with O-90 and O-81 curves than with O-72 and O-54 curves. 

Here's what 4" track spacing looks like when a 3rd Rail Allegheny passes on the inside of an 18" coach on the outside. The results are not pretty. I thought that since Ross track came in 4" radii increments that was the "correct" spacing. I was wrong! When i redesigned the layout I upped it 5" minimum, but basically keep them much further apart and on different levels whenever possible. I didn't want anything hitting anything else anytime, any where.

 

 

While 4" looks prototypical, it's operationally difficult with scale-length long equipment.

Every manufacturers track style presents different problems. 

 

I use MTH RealTrax. My curves are O-72 and O-54 exclusively.  This represents a 9 inch spacing everywhere between the outer track and the inner track. This spacing allows me to run very long passenger cars and 89 foot flatcars without problems with overhang on curves. I use O-72 crossovers between an outer O-72 track and an inner O-54 track. I also have an O-72 wye track entering my switchyard.

 

 

We are using 072 and 080 curves for the mainlines, and went with a 6" center to center spacing. Granted that is not required on straight track, but we have a lot of winding curves, several articulated engines, and a few full length (21") passenger cars. Clearance is NOT an issue with the 6" centers, for any of our equipment. We went with a 4" track center from any wall or bench work bracing with no issues. Everything was measured and designed for the BigBoy and 21" passenger cars....just in case

Looking at Lionel FasTrack, it's 6" centers due to the curves they have, and the new modules they sell kits for.  I've been planning a layout for my house I just got, and due to size limits, I'm using O-36 and O-48 for my curves with O-36 switches.  The switched put out to out also give 6: centers, but I can't speak for the larger radius switches.  To make sure I wouldn't have any issues with my Big Boy or my son's Amtrak HHP-8 train set, I put an O-36 curve on paper and rolled the Big Boy and Amtrak coach with a pencil on the corner that hung over the most on the inside and outside.  I measured and added it to a CAD model I had of the track (I'm a mechanical engineer so I planned my layout in SolidWorks during lunch hours.), and they cleared, just barely, but they did.