The primary purposes of NMRA STANDARDS are to establish the broadest correlated set of limiting dimensions, electrical parameters, and communications parameters within which interchange may be assured. This definition includes the intricacies involved in the Track-Wheel relationship for each scale defined. It is not the specific purpose of the NMRA STANDARDS to set production dimensions or tolerances but rather to set limits which manufacturers can use when setting their tolerances. When used to determine manufacturing tolerances care must be taken to ensure that the production dimensions are not at the extreme edge of the range specified. Specific NMRA Tech Notes supplement the standards to provide additional guidance.
The scale names, proportions and track gauges are provided in S-1. S-2 provides standards for couplers. S-3 and S-4 provide the critical wheel and track dimensions to ensure reliable operation
S1, S3, and S4 are subdivided into three classes: Proto Scales, Standard Scales, and Scales with Deep Flanges. The purpose of this division is to provide standards for the three general classes of scale model railroading.
To ensure interchange and reliable performance, NMRA STANDARDS S-3 and S-4 are designed so that Track and Wheels constructed within prescribed limits will meet the following related conditions. These conditions closely parallel prototype practices. Many of the various dimensions provided are strongly related to each other and care must be taken to meet each of the specifications provided.
1. TRACK Gauge (G in S-3) is the distance between railheads of two Stock (running) Rails of a length of Track.
2. TRACK CHECK GAGE (C in S-3) is the distance from the flange side of a Guard Rail to the flange side of the Frog it guards. WHEEL CHECK CAGE (K in S-4) is the distance from the Back of the flange of one wheel to the tread-side of the flange of the other wheel of a wheelset.
3. SPAN (S in S-3) is the distance between flange sides of the Guard and Wing Rails at the guarded Frog. BACK-to-BACK (B in S-4) is the distance between the Backs of the wheel flanges in a wheelset at railhead height.
4. FLANGEWAY WIDTH (F in S-3) is the distance between flange sides of the Wing and Frog Rails.TIRE WIDTH (N in S-4) is the distance between the Back of the wheel flange at railhead height and the outer edge of the wheel tread.
5. FLANGE CLEARANCE (H in S-3) is the vertical distance from the railhead to the highest obstruction below it.
FLANGE DEPTH (D in S-4) is the vertical distance from the root of the flange to its outer edge.
6. SWITCH POINT SPREAD (P in S-3) is the distance from the Gage Line of the closed Point Rail to the outside of the open Point Rail. See Fig 3.