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The NMRA has established a Recommended Practice, RP-11, to guide the modeler on what radius curves the various lengths of locomotives and rolling stock should have to work successfully. By the same token, there are standards (S-8 Track Centers) set for track centers in curves. Without getting too technical, I'll try to pass along some basics about both.
TRACK CURVATURE — RP-11
As an example, if you want to run all locomotives from large steam to any diesel, all passenger cars and all freight cars, the RP-11 minimum requirements would be a number 7 turnout with a 40" radius in HO or a 21.5" radius in N scale. Now, please note that I didn't say that you couldn't use smaller turnouts and curves. I said that this is the recommendation. There are reasons for this and if you've ever seen the overhang of an 86 foot boxcar or full sized passenger car on a tight radius curve, you will quickly understand. They look terrible and derailments can be common.
The era that you are modeling and the type of railroad can make all the difference in the world in your curves and turnouts. If you decide that your railroad is in the 40's or early 50's and you will never run passenger service, only use small steam and 4 axle first generation diesels and nothing larger than a 50' boxcar, you would be looking at Branch Line or Feeder Line service. This would allow number 5 turnouts and 16.5" radius in HO and 8.875" radius in N scale. Do you see the difference that the size of equipment can make? If you were to run street car traction lines, you could really pack some sharp curves into a small area, but with street cars and Interurbans, the couplers are different allowing for much greater swing.
One more example before we move on. Say you want to run steam locomotives s to 20' rigid wheel-base and diesel locos to 60' long with two 6-wheel trucks. You would need to use number 6 turnouts and 26.5" radius HO or 14.5" in N. This would also allow you to run passenger cars up to 70' long and freight cars up to 62' with no difficulty. Yes, you can fudge this and I have. I also have derailment problems, too. What I am recommending is that you take a look at RP-11 before planning that big layout and check your plans against curves and turnout sizes needed for the best operation.
Does this mean that you can't build a dream layout in a 4x8' space or even less? Not at all. I have seen a neat little HO layout built on a hollow-core door where small cars and locos are used and there is a fair amount of switching. No, what it does mean is that your equipment and available space should match for the best results.
For more information on Curvature and Rolling Stock, see RP-11.
TRACK CENTERS — S-8
To better understand Track Centers, I am going to use a question that I received several months ago about Standard S-8. To begin with, here are the Class definitions used for S-8 and I quote:
This STANDARD lists Track Center Distances and provides for Side Clearances required for various curvatures with three size categories of models.
Class II
Includes small four-wheel truck diesels, geared and other small steam locomotives with short end overhangs typical of old-time, logging and branch lines and equivalent rolling stock.
Class I
Includes longer steam locomotives typically with two-wheel trailing trucks, larger four and six-wheel truck diesels and equivalent rolling stock.
Class Ia
Includes the largest steam locomotives with four-wheel trailing trucks, articulated locomotives, those with wheelbases in excess of 20 feet, full length passenger cars and other long rolling stock.
Q) I have consulted the NMRA Web Page regarding track centers. I'm about to construct an HO scale railroad, employing first and second generation diesel locomotives (F units, GP & SD units, etc.) and "normal" 40' and 50' rolling stock. I'm concerned about double-track side clearance on a Class I.
I live beside the old Western Maryland Railway mainline and went out and measured its prototype double track centers, which, when scaled down to HO came out to exactly 2", within a few thousandths of an inch. The NMRA chart indicates that I should observe 2 5/16" track centers, and advised against 20" and 18" radius curves. Am I understanding this correctly?
A) Yes, you are correct about the prototype. You are also correct about the Standard. However, take a look at Class II figures. You are running shorter equipment and may be able to use Class II unless you are going to run passenger service. With passenger cars you will have potential side-swiping problems unless you open your curves.
Q) I guess my real dilemma is that I'm envisioning a suggested 2 5/16" double track center will look "non-prototype". Would I be living too dangerously if I kept my minimum radius greater than 20" and utilized a 2" track center dimension?
A) Very dangerously. Even the prototype opens the centers on tight curves for the same reason the S-8 Standard does. As a suggestion, do some testing. Use 2" centers and open to 2 1/16" on the curves. Get your longest equipment into the curves and test it. Just push them through. You don't even need power.
If that doesn't work, then use 2 1/16" on the tangent (straight) and open to 2 1/4" on the curves. Appearance is nice but we aren't the prototype. Our curves are much sharper than theirs. Notice that I opened the distance between the tracks to alter the appearance. If you do this, the wider distance in the curves won't look bad. Using 2" centers on the tangent and 2 5/16" on the curves will look really bad and you will not be happy with the results.
At this point our QnA took another turn that doesn't apply here. What does apply is that the sharper your curves are, the further apart your two tracks need to be depending upon the rolling stock that you wish to use. The fact is that the prototype (real 1:1) railroads do widen the distance between tracks on curves when they can especially now when freight cars have grown in both length and height. The longer that the car is, the more it will overhang when going around sharp curves. Not only on the ends, but this also applies on the inside of the curve as much as to the outside. I have broken a hi-level switch stand on a turnout because I didn't allow for the clearance to the inside of a curve for a long car. Our models generally require more side to side room both on the straights and on curves unless you are fortunate enough to be able run long sweeping turns and those number 8 turnouts that we would all like to have.
The NMRA Standards and RPs were developed to help modelers build layouts that would be as trouble free as possible.
For more information on Track Centers and the various Scales, see Standard S-8.
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Page last updated December 2, 2014