What is the NMRA?
The National Model Railroad Association, or NMRA, is a non-profit organization for those involved in the hobby or business of model railroading. It was founded in the United States in 1935, and is now active in Canada, Australia, Great Britain, and the Netherlands. It was previously headquartered in Indianapolis, Indiana, and is now based in Chattanooga, Tennessee. The best-known activity of the NMRA is the defining of standards, and advisory documents known as Recommended Practices (RP), for model railroad equipment. Many standards defined by the NMRA are widely followed by the industry and modellers, including their HO scale track and wheel standards (S-3, S-4) and related RPs (RP2, 8, 10-15, and 25) and their Digital Command Control (DCC) standard set (S-9.1, S-9.2, and RP9.1.1 through RP9.3.2). The RP25 wheel design in particular has been critical in ensuring the reliability and interoperability of US-prototype model railroad equipment, since practically every manufacturer of such
