NASG S Standards

The NASG maintains the track and wheel standards for the S-scale community. This page lists the latest accepted standards. Standards are maintained so that equipment from various manufacturers can run on track from various manufacturers. If you hand-lay track to these standards, the equipment made to these standards will run on your track.

Contact person: Gaylord Gill

"Scale" vs. "Gauge"

Throughout this web site we refer to modeling in 1:64th scale as "S-scale". You will find magazines and other web sites using the term "S gauge". That terminology is not correct, but it has its roots in the early 1900s when the hobby of model railroading started. You will find similar terminology used in other scales as well (e.g. "N gauge", "HO gauge").

The word "gauge" refers to the distance between the rails, whereas "scale" is the ratio of prototype (real-world) size to the modeled size. As a side note, the word "gage" means "pledge". So, "gauge" should be used to represent the distance between the rails, and the device by which such distance is measured.

There are two common uses of the word gauge that we use in model railroading in North America, namely, "standard-gauge" (4'8-1/2" of spacing between the rails) and "narrow-gauge" (e.g. 3 feet between the rails), yet locomotives that run on these rails are both scaled to 1/64th of the real thing.

Due to S-scale's history being tied to American Flyer, there is further confusion in that, within the S-scale community, there are sub-divisions, such as American Flyer, hi-rail, scale, and narrow-gauge. Some refer to "S gauge" as meaning either American Flyer and/or hi-rail, and "S scale" as being scale modeling (standard-gauge or narrow-gauge). This is incorrect, because all models are still 1/64th of the real thing, so all are indeed S-scale.

If the distinction needs to be made between "American Flyer", "hi-rail", or "scale" modeling, it will be so indicated on this site using those words as shown in these quotes. The NMRA standard uses the term "deep-flange", which would apply to S-scale's American Flyer and hi-rail modeling (anything that uses or requires code 125 or taller rail).

Within S-scale, the difference between all of these sub-divisions really only boils down to whether or not your equipment uses deep flanges and/or American Flyer-compatible claw couplers, versus more to-scale flanges and the smaller couplers. Other distinctions, such as level of detail on the models, and whether or not animated accessories are used, are independent of these definitions and are really a personal decision the modeler makes to enjoy his or her involvement in the hobby.

Past NASG presidents Roy Hoffman and Bill Winans have publicly expressed that we should start to use the term "S-scale" instead of "S gauge". Also, past NASG Vice-President Dave Heine wrote an article in the February 2009 issue of the NASG Dispatch (pg 20), explaining the difference between scale and gauge.

About the Standards

The prototype for these standards are North American rail, where the standard-gauge railhead spacing is 4 feet, 8-1/2" (56.5", 143.51cm). Narrow-gauges covered here are Sn3 (3 feet, 36", 91.44cm), and Sn2 (2 feet, 24", 60.96cm). Sn42 (a.k.a Sn3-1/2, 42", 106.68cm), which is also modeled (using HO-scale track), appears to not have any formal standards.

The current NMRA standards for S-scale (1:64) models are identical to the NASG's standards. If you visit the NMRA web site, its documents list the standards for all scales. What we have done on this page is simply filter out only the ones specifically for S-scale.

The NMRA standards that start with "S-" are model railroading industry standards as maintained by the NMRA and NASG. The ones starting with "RP-" are "recommended-practices", which are not standards, but are commonly followed nonetheless.

About the NASG Standards

NASG S-1.1

It is the official position of the NASG that the code of the rail (also known as the rail height in 1,000^ths of an inch) and the code of the wheels (also known as the width or thickness in 1,000^ths of an inch) shall be used together to define the three categories of S:

• American Flyer:
     rail: code 172
     wheels: code 220 (American Flyer) and code 172 (Hi-rail)
• Hi-rail:
     rail: code 125 through 148
     wheels: code 220 (American Flyer) and code 172 (Hi-rail)
• Scale:
     rail: code 55 through 125
     wheels: code 110 or smaller

Exceptions
The exceptional case where modelers run scale wheels on code 138 rail employing scale flangeways will be considered "Scale".

Caution
In general, rolling stock using "American Flyer" and "Hi-rail" rail codes and wheel profiles are incompatible with "Scale" rail codes unless custom-made turnouts (switches) with movable frogs are employed.

The chart below demonstrates the above definitions. Click the image for a larger view.

Standards Categories

Click the link of interest below, or just scroll down the page.

• General Track (S-1)
• Couplers (S-2)
• Turnouts (S-3)
• Wheels (S-4)
• Traction (S-5)
• Interurban (S-6)
• Track Clearance (S-7)
• Track Centers (RP-7.2)
• Three-point Track Gauge (RP-8)
• Car Weight (RP-20.1)
• Modules
• Gauges (tools)

S-1 General Track

Straight Track

This table shows the minimum and maximum measurements of the distance between the railheads of general straight track for S-scale (denoted as "G" in diagrams). For hi-rail/deep-flange, minimum rail size is code 125 (see NMRA S-1.3). Note that for some uses, the gauge-maximum is different at the turnout frog than it is elsewhere (minimum is always the same).

RP-11 Minimum Track Curvature

This RP deals with the minimum track curvature recommended for the given equipment to be run on the track. "Model Radius" are the actual measurements for our S-scale models.

Street cars:

Interurbans (in-city-streets):

Interurbans & trailers up to 40':

Interurbans & trailers up to 50':

Interurbans & trailers up to 64':

Electric up to 50' passenger to 60' (no diaphragms) freight to 50':

Electric up to 60' passenger to 60' (diaphragms):

Electric up to 70' passenger to 70':

Small steam/diesel with 4 wheels cars to 40':

Steam up to 11' rigid wheel-base diesel up to 40' (4-wheeled) cars up to 50':

Steam up to 14' rigid wheel-base diesel up to 50' (4-wheeled) cars up to 60':

Steam up to 17' rigid wheel-base diesel up to 60' (4-wheeled) passenger up to 60' freight up to 50':

Steam up to 20' rigid wheel-base diesel up to 60' (4-/6-wheeled) passenger up to 70' freight up to 62':

Steam up to 24' rigid wheel-base diesel up to 70' (4-/6-wheeled) passenger up to 80 freight up to 85':

Steam up to 28' rigid wheel-base:

RP-15.1 Rail

This NMRA RP deals with the actual shape and dimensions of the rail. It contains the actual dimensions for the individual parts of a piece of rail (rail head height, curvature, etc.), providing dimensions for codes 32 through 297. Those are probably only useful to manufacturers of model rail.

However, that RP also provides a table that matches recommended scale rail codes to the prototype type of rail modeled. We have reproduced the S-scale portion of that list below. The definition of the columns are as follows:

Prototype Weight: In the real world, rail is measured by weight, and so the numbers shown below indicate the number of pounds per yard (~1 meter) of rail.

Prototype Height: The physical height, in inches, of the rail, which is typical for that size of rail.

Model Rail: in model railroading, "code" represents the height of the rail in terms of 0.001 (one-thousandths-of-an-inch). So, "code 125" is actually 0.125 inches, or 1/8", tall. Within S-scale, typical rail codes used are 100, 88, 70, 55, and 40 for "scale" standard-gauge and narrow-gauge modeling. For S-scale, the NMRA defines deep-flange (a.k.a. hi-rail) as anything that uses code 125 or taller. Of course, choice of rail height is the modeler's prerogative, usually based on wheelsets used, prototype railroad modeled, etc. This listing is simply provided as reference.

Ties

In North America, standard-gauge track uses wooden railroad ties which measure 8'6" (2.59m) in length, 7" (17.78cm) tall, and 9" (22.86cm) wide. The distance between the ties is typically 21" (53.34cm), so with 9"-wide ties that makes the gap between ties 12" (30.48cm). This equates to 3,000 ties per mile (1.604km).

Wooden ties provide "give" which makes the rail and the equipment riding on the rail last longer. Wooden ties are impregnated with a creosote preservative so that they can last 25 to 30 years. They weigh around 225lbs (100kg) each. Concrete ties are being used in newer track in North America. Their "give" is between the ties and the ballast. They weigh around 700lbs (318kg) each.

References

• NMRA S-1.1 Standards for Proto Models covers standard- and narrow-gauge proto:64 measurements (PDF).
• NMRA S-1.2 Standards for Scale Models covers standard- and narrow-gauge general measurements (PDF).
• NMRA S-1.3 Standards for Deep Flange Models covers hi-rail/deep-flange general measurements (PDF).
• NMRA RP-10 Trackwork General (PDF).
• NMRA RP-11 Curvature and Rolling Stock (PDF).
• NMRA RP-15.1 Rail (PDF).
• "The Railroad What It Is, What it Does" by John H. Armstrong.
• Demystifying The NMRA Standards (four-part video series by FastTracks).

S-2 Couplers

This table shows the coupler height for S-scale, as measured from the top of rail to the center of the knuckle. The prototype has a tolerance of +/-1.5 inches (3.81cm).

References

S-3 Turnouts

The dark-colored rails represent the stock rails. The letters in the diagram represent measurements that are provided in the tables, and only applies to non-proto:64 standards. For "G", which is the general track spacing, see S-1 above (specifically its third data column).

Standard Gauge

Hi-rail

Narrow-gauge (Sn3)

Narrow-gauge (Sn2)

Proto:64

The diagram below applies only to proto:64 standards for standard- and narrow-gauge tables below. For "G", which is the general track spacing, see S-1 above (specifically its third data column).

Narrow-gauge (Sn3) Proto:64

RP-12 Turnout Dimensions

This RP deals with the actual dimensions of the various pieces and rails that make up a turnout. The NMRA has documents showing the individual measurements, labeled in the diagrams below, for both straight and curved turnouts, in frog numbers from 4 through 20. The documents for S-scale are copied here to our web site, because the NMRA keeps them in one large .zip file for all of the scales together. So, rather than having to download that entire file, and then needing to extract the one or two documents that you actually want, you can download them directly from our web site (see links below).

References

• NMRA S-3.1 Standards for Proto Models covers standard- and narrow-gauge proto:64 measurements (PDF).
• NMRA S-3.2 Standards for Scale Models covers standard- and narrow-gauge general measurements (PDF).
• NMRA S-3.3 Standards for Deep Flange Models covers hi-rail/deep-flange general measurements (PDF).
• NMRA RP-12 Turnouts - General: general, non-scale-specific information (PDF).
• NMRA RP-13.5 Guard Rail & Frog Relationship: provides more details about diagram item #30 (PDF).
• NMRA RP-13.6 Guard Rails: duplicates guard rail dimensions #32, #33, #34 (PDF).
• NMRA RP-13.7 Frog & Wing Rails: duplicates frog and wing rail dimensions #19, #20, #21, #22 (PDF).
• NMRA RP-13.8 Flangeway Flares: provides more details about how to form flangeway flares (PDF).
• Fast Tracks standard-gauge Templates: turnouts, wyes, 3-ways, crossings, and slip switches.
• Fast Tracks narrow-gauge (Sn3) Templates: turnouts, wyes, 3-ways, crossings, and dual-gauge.
• Fast Tracks narrow-gauge (Sn2) Templates: turnouts and wyes.

S-4 Wheels

The letters in the diagram represent measurements that are provided in the tables.

Standard Gauge

Hi-rail

Narrow-gauge (Sn3)

Narrow-gauge (Sn2)

Proto:64

Narrow-gauge (Sn3) Proto:64

RP-23 Bolsters

This RP deals with the dimensions of trucks. The purpose is to make it easy to interchange trucks between cars. The diagram below applies to the S-scale measurements listed below it.

(P) Kingpin Size: 0.112" (#4)

(E) Truck Bolster Hole: 0.116 (#32 Drill)

(D) Center Plate Diameter: 3/8" max.

(Bc) Center Bolster Bearing (freight): 13/32"

(Bc) Center Bolster Bearing (passenger): 1/2"

(Bs) Side Bolster Bearing: not recommended

(Si) Distance Side (inside): not recommended

(So) Distance Side (outside): not recommended

RP-24.1 Journals

This RP deals with the dimensions of journals. The diagram below applies to the S-scale measurements listed below it.

(J) Journal Bore min. diameter: 0.067"

(V) Journal Bore min. depth: 0.123"

(X) Journal Bore min. span: 1.314"

(W) Journal Face min. spacing: 1.068"

RP-24.3 Axles

This RP deals with the dimensions of axles. The diagram below applies to the S-scale measurements listed below it. The standard lists this specific note for S-scale: "Axle insulating bushings are optional, and, if used, may extend beyond the outer face of the wheel. The maximum allowable distance over the outer faces of bushings shall be 1.064 inches."

(A) Axle Diameter max.: 0.063"

(Y+) Shoulder min.: 3/32"

(U) Axle Length max.: 1.310"

References

• NMRA S-4.1 Standards for Proto Models covers standard- and narrow-gauge proto:64 measurements (PDF).
• NMRA S-4.3 Standards for Deep Flange Models covers hi-rail/deep-flange general measurements (PDF).
• S-Helper Service's Hi-rail Wheel Contour (.gif), provided by Don Thompson.
• S-Helper Service's Hi-rail Wheelset (.gif), provided by Don Thompson.
• NMRA RP-21.2 Solid Couplers (PDF); probably only of interest to coupler manufacturers.
• NMRA RP-23 Bolsters (PDF).
• NMRA RP-24.1 Journals (PDF).
• NMRA RP-24.3 Axles (PDF).
• NMRA RP-25 Wheel Contour (PDF).

S-5 Traction

This section concerns itself with standards for traction motive power.

Overhead Wire

Trolley Wheel

The diagram below applies to the S-scale measurements listed below it.

Third-rail

The diagram below applies to the S-scale measurements listed below it.

References

• NMRA S-5 Traction Power Collection (PDF).

S-6 Interurban Clearance & Track Centers

These clearances and track centers provide for interchange and satisfactory operation of interurban and street railway equipment only. The outside clearance is the amount of space to leave unobstructed from the outside-of-the-curve rail out. The inside clearance is the amount of space to leave unobstructed from the inside-of-the-curve rail out. Track center is measured between two tracks' centerlines (midway between the rails). All dimensions given apply to S-scale models only.

References

• NMRA S-6 Interurban Clearance & Track Centers (PDF).

S-7 Track Clearances

This standard specifies the clearances for bridges, structures, and tangent tracks for Old Time (pre-1920), Classic (1920-1969), and Modern (post-1983) eras. The NASG sells clearance gauges for the "Classic" and "Modern" eras (click on "Store" on the left, then "NASG Company Store").

RP-7.2 Track Centers

This standard specifies the distance between two tracks' centerlines (midway between the rails). Curved track is assumed to be eased. "Actual" is the actual measurement for S-scale models. The NMRA used to have this flagged as section S-8 of the standard, but sometime in 2020/2021 they changed it to "RP-7.2", because a "standard" is one that is required to make a train operate on a layout, whereas track centers do not necessary require to be accurate to be able to operate a layout (e.g. one train running on a double-tracked layout where the curves are too close to each other; that one train can still run just fine). Hence, it is now considered a "recommended practice".

Standard Gauge

In the standard-gauge table below, the following represents the kind of equipment presumed to be running on the tracks.

• Class II:
  diesel: small 4-wheeled
  steam: small, geared (logging, branch, old-time).
• Class I:
  diesel: large 4-, 6-wheeled
  steam: longer (x-x-2).
• Class Ia:
  diesel: (any)
  steam: larger (x-x-4)
  cars: articulated passenger.

^* Not prohibited, but also not recommended.

Narrow-gauge (Sn3)

RP-8 Three-point Track Gauge

The design specifications for a three-point track gauge. So far only the standard-gauge version has been defined and standardized. See the previous section for the definitions of "Class II", "Class I", and "Class Ia". For "G", which is the general track spacing, see S-1 above.

For Class II, the gauge length (L) at a 25" radius is 1-7/8".

For Class I, the gauge length (L) at a 31" radius is 2-3/32".

For Class Ia, the gauge length (L) at a 43" radius is 2-7/16".

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References

• The NMRA RP-2 Track Gauges (PDF).

RP-20.1 Car Weight

Standard-gauge

The S-scale car weight standard is as follows:

A couple of examples are:

A standard 40-foot box car should weigh 5.75 ounces (2oz + (7.5" x 0.5oz)), or 163 grams for a 19.1cm car.

A standard 50-foot reefer should weigh 6.7 ounces (2oz + (9.375" x 0.5oz)), or 190 grams for a 23.8cm car.

Narrow-gauge (Sn3)

The car weight standard is as follows:

References

• The NMRA RP-20.1 Car Weight (PDF).

Modules

For any of our S-MOD documentation, see our dedicated S-MOD Modules page.

For any of the S-scale version of the T-TRAK system, see our dedicated T-TRAK Modules page.

For any of the S-scale versions of the Free-mo system, see our dedicated Free-mo Modules page.

Gauges

To aid in enforcing or implementing the above-mentioned dimensions, there are several gauges available on the market. The NASG sells several, which you can find in the Store section of our web site. You can view the enclosed instructions sheet here (PDF). The NMRA also carries our standard-gauge and Sn3 gauge, for those of you who are NMRA members. Finally, there are several manufacturing companies that offer gauges of different shapes and styles, such as Tomalco Track, Railway Engineering, P-B-L, CESCO (Sn3, via Shapeways), Copper Range Products (Sn2, via Shapeways), Clover House, Crusader Rail Services, Fast Tracks, Great Decals!, etc.

As a historical note, when the current NASG steel wheel-and-track gauge was first manufactured, the company that did the actual work, printed the text on the gauge in mirror-image. The gauge itself was perfectly fine; one just had to ignore the texts. A number of them went out into the public, but they were marked with a permanent marker with an "X" on them, as is shown in the photo below. These are no longer being sold, but you might still stumble across one in an estate sale. At least now you know what they are.