The best-known activity of the NMRA is the defining of standards, 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 (RP25) and their Digital Command Control (DCC) standard set. The RP25 standards in particular have been critical in ensuring the reliability and interoperability of US-prototype model railroad equipment, since practically every manufacturer of such equipment follows the RP25 standards. This is in marked contrast to the British market, in which there is no accepted standard among manufacturers. In Europe, the NEM standard is widely followed, but this is further away from prototype accuracy than the RP25 standard.
The NMRA also considers the education of model railroaders and encouraging learning and improving modellers' skills to be part of its mission. The NMRA runs an achievement program to encourage these skills, and runs modelling contests.
Promotion of the model railroading hobby is also part of the NMRA's purpose.