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.
The NMRA came into being in 1935 with a gathering of model railroaders, manufacturer's, and publishers, in response to the need to bring order out of chaos. The NMRA Standards were developed as a way to help insure that equipment could be interchanged between one model railroad and another and that cars and locomotives of one manufacturer could run on the track of another manufacturer together with cars and equipment of still other manufacturers and modelers.
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.
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 from time to time but ONLY by the vote of the NMRA membership after proper policy procedures have been followed and the membership has been fully informed.
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 not required to be voted by the membership except when in a package such as the Module Standards and RPs. These are presented to the Board of Trustees (BOT) for their study and 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.