Scale for Locomotives/Rolling stock, includes Track gauges, and rail codes.

Prototype: - An original full size item.
Scale modeling: - Miniaturization of an original full size item to a desired scale.
S Scale: - 1/64th or (3/16" = 1') reduction in size of the prototype (proto).
S Gauge: - Track width, inside edge to inside edge of rail heads, .883-.905, AKA 7/8".
Rail code: - Height of rail in thousandths.

Scale and Gauge! Scale being the reduction in size of the prototype of everything on your layout when modeling to a desired scale (miniaturization). However, gauge is the established width between rails of prototype or modeled track. To establish track gauge, measure perpendicularly across the inside edges of the railheads. Track height and weight vary in prototype and modeled track as to the main line, yard, branch and spurs. In order to effectively compliment track realism, modeling vendors have specialty track, turnouts and crossovers for this purpose.

Model railroad type example Sn3.5 is as follows, S (scale) being the prefix to n (narrow gauge) followed by 3.5 (prototype track gauge in feet), 3 and 1/2 feet, or 42 inches, which in Sn3.5 equates to actual modeling track width of .649 - .672 thousandths, 16.5 - 17.1 mm. Track gauge specifications including rail height codes, see table below.

Difference in "Standard gauge prototypes" and "Narrow gauge prototypes"? 1800s and early 1900s "narrow gauge prototype" locomotives and rolling stock are small when compared to the later 1900s "standard gauge prototype" locomotives and rolling stock. The track gauge for "narrow gauge prototype" locomotives generally measured from 24" to 42", where the "standard gauge" for the larger style later 1900s prototype locomotives and rolling stock would have been 56 -1/2". When modeling "narrow gauge", "Sn", the down-size difference from "standard gauge" to "narrow gauge" is still 1/64th scale. Locomotives and rolling stock operating on the narrower variations of track gauge will have rail height and rail weight defined to suit the narrow gauge selected. Structures and compliments to the layout are always full S scale.

Guidelines for "S" model railroading. The scale of everything in the layout including locomotive and rolling stock body shells, wheel diameters etc, are S scale, being 1/64th prototype whenever possible. The functioning mechanical equipment in contact with the rails as to gauge shall be sized to match the intended S gauge track of (.884-.905) thousandths. Use rail height codes and set coupler heights as required. Structures are always S scale.

Guidelines for "Sn" verities of model railroading. The scale of everything in the layout including locomotive and rolling stock body shells, wheel diameters etc also are "S" scale at 1/64th of prototype whenever possible. The functioning mechanical equipment in contact with the rails as to the narrow gauge version selected shall be sized to match the equivalent track gauge modeled, (Sn3.5 .649-.672), (Sn3 .563-.586), (Sn2.5 and Sn30 wide track versions .500-.522), (Sn2.5 and Sn30 narrow track versions .471-.483), (Sn2 wide track version .413-.424) and (Sn2 narrow track version .353-.367) in thousandths. Sn locomotives and rolling stock may be bought, built or modified (up or down) from scales smaller than S scale (On3, HO, On2, TT, HOn3 and N), modify, (aka bashing) requires sizing of locomotives, rolling stock and driven wheels, trucks, and couplers to compliment your desired period and gauge. Use correct rail height codes and set coupler heights as required. Structures used in narrow gauge versions are still always S scale.

Narrow gauge modelers gravitate to the 19th and early 20th century layouts to create for themselves historically correct prospectives as to the period they present, while modeling to a museum quality standard.

S Scale Types Track Gauge Specifics Available
Rail Code Heights
(in thousandths)
S Scale
S Narrow
Track
Scale
Track
Guage
Min/Max
Inches
Min/Max
mm
Guage
AKA
Proto
Guage
AF 1/64 S .884 - .905 22.4 – 23.99 7/8" 56.5" .131 .148 .172 .220
S 1/64 S .884 - .905 22.4 – 23.99 7/8" 56.5" .070 .083 .100 .125
P64 1/64 S .884 - .905 22.4 – 23.99 7/8" 56.5" .070 .083 .100 .125
Sn4 1/76 On3 .750 - .772 19.1 – 19.8 3/4" 48" .070 .083 .100
Sn3.5 1/87 HO or .649 - .672 16.5 – 17.1 5/8" 42" ** .070 .080 .083 .100
Sn42 1/87 On30 .649 - .672 16.5 – 17.1 5/8" 42" ** .070 .080 .083 .100
Sn3 1/100 Sn3 or .563 - .586 14.3 – 14.9 9/16" 36" * .055 .070 .083 .100
P64n3 1/100 OOC .563 - .586 14.3 – 14.9 9/16" 36" * .055 .070 .083 .100
Sn2.5(1) 1/114 On2 .500 - .522 12.7 – 13.3 1/2" 30" .040 .055 .070 .083
Sn2.5(2) 1/120 TT .471 - .483 12.0 – 12.3 12mm 30" .040 .055 .070 .083
Sn30(1) 1/114 On2 .500 - .522 12.7 – 13.3 1/2" 30" .040 .055 .070 .083
Sn30(2) 1/120 TT .471 - .483 12.0 – 12.3 12mm 30" .040 .055 .070 .083
Sn2(1) 1/140 HOn3 .413 - .424 10.5 – 10.77 10.5mm 24" .040 .055 .070 .083
Sn2(2) 1/160 N .353 - .367 8.97 –  9.32 9mm 24" .040 .055 .070 .083
(1) Wide track version.   (2) Narrow track version.   P = Proto   Hi-Rail Scale (Red). * Also Code .040 ** Also Code .055

To those of you, who are just entering S scale, "Welcome to the neighborhood".

Why consider S scale/gauge against other popular scales/gauges above and below S scale/gauge? Comparing gauges O, S and HO, we have O gauge being approximately 30% larger than S gauge, and HO gauge being approximately 28% smaller than S gauge. That puts S gauge right in the middle, track wise! Ok, now to be fair, let's look at S scale against other popular scales above and below S scale. Comparing scales O, S and HO, we have O scale being approximately 25% larger than S scale, and HO scale being approximately 26% smaller than S scale. That puts S scale right in the middle also. The margin for error is approximately 3% or less. Everything is generally relative, including layout space and the minimum track radiuses required fall between O and HO's requirements.

Having the ability to model without utilizing precious space, modeling with attention to detail which was unachievable in HO, having access to die cast AF models which are modifiable to the lower rail code heights, these are just a few of the many advantages in scale S. There are plenty of manufacturers when it comes to structures, track and accessories, bridge and tunnels, trackside signals etc. New locomotive and rolling stock manufacturers are somewhat limited when compared to O or HO. The required items for a quality layout are generally less expensive than O and a bit more expensive than HO. "Scale S, the user friendly scale."

Basic list of items needed to create an S or Sn scale layout
Be Informed!
Layout Software
Layout Board Construction
Topographical Details
Background Details
Layout Wiring, Excellent Site
Controller Size and Type
Track and Accessories
Rigid Track
Flex Track
Terminal Connector
Turnouts
Uncoupler
Track Guage
Switch Machines
Crossing
Wye ("Y")
Road Bed
Rail Joiners
Spikes
Ballast
Grade Crossing
Turn Table
Roundhouse
End Bumper Post
Signal Switching Devices
Spanning and Tunneling
Trestle, Timber
Trestle, Riveted Steel
Trussed Under, Deck, Girder Bridge
Trussed Parallel Side, Girder Bridge
Trussed Arched Side, Girder Bridge
Riveted Steel Side Plate, Girder Bridge
Overhead Through Truss, Girder Bridge
Draw Bridge
Bascule Bridge
Bridge Support Piers
Tunnel Portals
Signal Bridge
Trackside
Station Depot
Water Tank
Coaling Station
Freight Platform
Platform Ramp
Lamp Posts
Telegraph Electric Poles
RR Employee Life-Like Scale Figures
RR Repair Rolling Stock, on Siding
RR Owned Highway/Track Vehicles
Tool Shed
Express Carts
Trackside Signals
Grade Crossing Crossbuck Signs
Grade Crossing Crossbuck Signals
Gated Crossing Crossbuck Signals
Semaphore Signals
Dwarf and Switch Signals
Single Target Mast Signal
Double Target Mast Signal
Triple Target Mast Signal
Bi-Directional Mast Signal
Cantilevered Signals
Domino Signals
Single Searchlight Signal
Double Searchlight Signal
Triple Searchlight Signal
Signal Electric Panel Boxes
Surrounding Community
Business Structures
Industrial Structures
Housing Structures
Community Structures
Life-Like Scale Figures
Scale Vehicles
Streets
Landscape
Fire Hydrants