Before the 1930s, there were no common standards pertaining to model railroad equipment. One manufacturer's equipment would not necessarily work with another manufacturer's or even run on someone else's track. Many modelers built to their own standard or from their own designs and ideas. In many cases it was difficult, if not impossible, to take your cars or locomotives to another modeler's railroad and expect them to run without problems. There were nearly as many couplers as there were manufacturers. This situation could only work to the detriment of the hobby as a whole.

In the early 1930’s, model railroaders were starting to discuss the need for standards on flange and wheel dimensions, the location of third rail, and other technical issues impacting the interchangeability of model railroad equipment. The lead editorial in the first issue of “The Model Railroader” in January 1934 was dedicated to this topic. By the summer of 1935 many were discussing the formation of a national organization to develop standards. The Model Railroad Club of Milwaukee decided to host a national convention during Labor Day weekend in 1935. During the convention a proposal was adopted to organize a national association, although not all in attendance were in favor. There, on September 1, 1935, the NMRA was officially formed to develop model railroad standards and promote fellowship among model railroaders. There was concern it was too early to form a national association, but those in favor held the majority. Many national convention precedents were established at this first meeting, such as a guest speaker, prototype tours, business meetings, and the installation of association officers.

Since 1936, many of these basic Standards have remained virtually unchanged from the time of their original publication. They have been added to and refined, but they have stood the test of time and have proven to have been of great benefit to the hobby of model railroading and have contributed greatly to allowing the hobby to develop to the point where it is today.

What Is a Standard and How Does It Differ from a Recommended Practice?

NMRA Standard S-1 states that the purpose of the standards “are to establish the broadest correlated set of limiting dimensions, electrical parameters, and communications parameters within which interchange may be assured.”

A Standard is a figure, relationship or dimension that is mandatory, it is "cut in stone" so to speak and must be followed to facilitate interchange or interface, whichever the case may be. Standards can be changed but ONLY after the change has been reviewed by the membership, comments from the membership reviewed and where appropriate incorporated and then presented to the Board of Directors for approval.

RPs (Recommended Practices) are those figures, relationships or dimensions that the Engineering Committee has established through actual tests and feel are beneficial to operation.  These are presented to the Board of Directors (BOD) for their approval.

As charged by the NMRA REGULATIONS, NMRA STANDARDS provide the primary basis upon which Interchange between equipment and various North American scale model railroads is founded. Under this requirement NMRA STANDARDS include only those factors that are considered vital to such Interchange. For less critical matters see the NMRA RECOMMENDED PRACTICES.


Upon official Inspection by NMRA, those items found to be in Conformance to all NMRA STANDARDS, applicable NMRA RECOMMENDED PRACTICES, and industry quality metrics, may be recognized by the display of the Conformance/Inspection Seal of NMRA in their advertisements and on-their packages as authorized by the Certificate of such Conformance awarded their manufacturer.

The NMRA uses five types of Technical Documents to present information.

Standard (S) - NMRA Standards are to establish the broadest correlated set of limiting dimensions, electrical parameters and communications parameters within which interchange may be assured. STANDARDS category is considered mandatory for acceptable interoperability. Standards are reviewed by the membership and approved by the Board of Directors. 

Recommended Practice (RP) - NMRA Recommended Practices are only less mandatory than Standards by virtue of their slightly less critical subject matter with respect to interchange and/or the fact that deviation for specific reasons is permissable. Recommended Practices do not rise to the level of being a Standard, however, they provide details of various topics that have been found to be the best for interchangeability or operation of products.  Recommended Practices are not used in determining the qualification of a product when compliance certification is considered.  Comments are generally made if the product is found outside the recommended practice.  RECOMMENDED PRACTICE category was established by the NMRA Board of Trustees in January 1957, to specify the details of major components to improve design and function, and to promote maximum interchange between and withjin units. Recommended Practices are reviewed by the membership and approved by the Board of Directors.

Technical Note (TN) - NMRA Technical Notes supplement the Standards to provide additional guidance on a given subject. Technical Notes are reviewed by the appropriate technical working group and approved by the Manager, Standards & Conformance.


Technical Information (TI) - NMRA Technical Information are documents that are used to provide information associated with a Standard or Recommended Practice.  Technical Information is reviewed by the appropriate technical working group and approved by the Manager, Standards & Conformance.

Technical References (TR) - NMRA Technical References are, for the most part, internal documents that provide a history of action taken in the Standards and Conformance Department.  Occasionally a Technical Reference is made public for reference and use by individuals involved with the Standards and Conformance Department.  Technical References are reviewed by the appropriate technical working group and approved by the Manager, Standards & Conformance.

A standard Template is used to generate Technical Documents.

The Standards and Conformance Department has adopted a standard format for Technical Documents. This standard format is provided in the form of a Microsoft Word dotx template, which can be found here at TR-1-2020 templateInstructions for use of the Template can be found here at Template Instructions (TR-1-2020).

Documents released for NMRA Membership Comment.

DRAFT revisions of DCC Standards by the NMRA DCC Working Group may be found in the DCC section of this website for general membership comment prior to presentation to the Board of Directors.    Send comments to tech-chair [at] and dcc-manager [at] Questions and comments on Standards and Recommended Practices on this page may be address to tech-chair [at] or CIManager [at] NMRA.ORG 

Legal Notices and Disclaimers

Legal Notices and Disclaimers for the use of NMRA Standards, Recommended Practices, Technical Notes, Technical Information, and Technical Reference documents are available at this link:  Legal Notices and Disclaimers.

Updated  29-December-2023