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NMRA Standards and Recommended Practices

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

The NMRA, in its standards and recommended practices, addresses only the specific standards and recommended practices for model railroads.  These standards and recommended practices do not address any legal requirements.  It is the responsibility of the manufacturer to determine if their product conforms to all applicable governmental laws and regulations.

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. 

Recommended Practice (RP) - NMRA Recommended Practices are only less manditory 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 provide details of various topics that have been found to be the best for interchangeability or operation of products.  

​Technical Note (TN) - NMRA Technical Notes supplement the Standards to provide additional guidance on a given subject. 

 

Technical Information (TI) - NMRA Technical Information are documents that are used to provide information associated with a Standard or Recommended Practice.  

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.  

Standards, Recommended Practices, and Technical Notes are divided into nine (9) categories:

Category Description
1.n Overview of General Standards
2.n Couplers
3.n Trackwork
4.n Wheels
5.n  Traction - Power Collection
6.n formerly Traction Clearances & Track Centers
7.n formerly Clearances
8.n formerly Track Centers
9.n Electrical, Digital Command Control, Layout Command Control

Legal Notices and Disclaimers

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

Standards (S)

S-1 S-1 General Overview - (4/2021)  
   RP-1 ​RP-1 Abbreviations - (4/2021)  
   S-1.1 General - Proto Scales - (7/2009)  
   S-1.2 General - Standard Scales - (7/2009)  
   S-1.3 General - Scales with Deep Flanges - (7/2009)  
S-2 Couplers - (7/2010)  
S-3 Trackwork  
   S-3.1 Trackwork - Proto Scales (1/2006) TN 3.1 PROTO:SCALE AND FINE SCALE GUIDE - (11/22/2005)**
   S-3.2 Trackwork - Standard Scales - (5/8/2010)  
   S-3.3 Trackwork - Deep Flanges - (2/24/2010)  
S-4 Wheels  
   S-4.1 Wheels - Proto Scales - (1/2006) TN 4.1 PROTO:SCALE AND FIND SCALE GUIDE - (11/22/2005)**
   S-4.2 Wheels - Standard Scales - (01/2019)  
   S-4.3 Wheels with Deep Flanges - (2/2010)  
S-5 S-5 TRACTION Power Collection - (8/1982)  
S-9 S-9 ELECTRICAL - (1984)
♦ Draft Revisions		 TN-9 Wiring for DC and DCC  (12/30/2023 Revised)
  DCC Section  
   S-9.1 S-9.1 DCC Electrical Standard - (rev. 4/9/2021)  
  S-9.1.1  Decoder Interfaces  (1/31/2021) TI-9.1.1 Sources for Connectors for DCC Decoders - (11/19/2021)
  S-9.1.1.1  Six and Eight Pin Decoder Interface  (11/13/2020)  
  S-9.1.1.2  JST-9 Decoder Interface   (11/13/2020)  
  S-9.1.1.3  21 MTC Decoder Interface  (5/6/2024) TN-9.1.1.3 21 MTC Decoder Interface (12/1/2020)
  S-9.1.1.4 PluX Decoder Interface (7/7/2021) TN-9.1.1.4 PluX Decoder Interface (6/12/2021)
  S-9.1.1.5  Next 18 & Next 18S Decoder Interface  (12/6/2020) TN-9.1.1.5 Next 18 & Next 18S  Decoder Interface (9/10/2020)
  S-9.1.2 DCC Power Station Interface - (6/9/2021) TN-9.1.2 Power Station Interface (6/9/2021)
   S-9.2 S-9.2 DCC Communications Standard - (2004)  
  S-9.2.1 DCC Extended Packet Formats - (5/15/2022)
♦ Draft Revision		  
  S-9.2.1.1 DCC Adv Extended Packet Formats - (5/15/2022)
♦ Draft Revision		 TN-9.2.1.1 DCC Adv Extended Packet Formats - (5/15/2022)
  S-9.2.2 DCC Configuration Variables - (2012)
♦ Draft Revision		  
  S-9.2.2 Appendix A, DCC Mfg ID - (12/10/2024)  
  S-9.2.3 Service Mode Programming - (Under revision)  
  S-9.2.4 DCC Fail-Safe - (7/2012)  
   S-9.3 DCC Bi-Directional Communications  
  S-9.3.1 (discontinued)  
  S-9.3.2 DCC Basic Decoder Transmission - (12/20/2012)
♦ Draft Revision		  
9.7 Layout Command Control (LCC)  Section Standards Revised 20-Feb-16 & 25-Apr-21
  ♦ Draft Revisions Complete set of new drafts 22-Jul-24
  9.7.0 DEFINITIONS & IDENTIFIERS  
    TN-9.7.0.1 Glossary
    TN-9.7.0.2 Common Information
  S-9.7.0.3 Unique Identifiers  TN-9.7.0.3 Unique Identifiers
  S-9.7.0.4 Event Identifiers TN-9.7.0.4 Event Identifiers
  9.7.1 PHYSICAL LAYER  
  S-9.7.1.1 CAN Physical TN-9.7.1.1 CAN Physical
  9.7.2 DATA LINK LAYER  
  S-9.7.2.1 CAN Frame Transfer TN-9.7.2.1 CAN Frame Transfer
  9.7.3 NETWORK & TRANSPORT LAYER  
  S-9.7.3 Message Network (CAN + TCP/IP) TN-9.7.3 Message Network (CAN + TCP/IP)
  S-9.7.3.1 Event Transport TN-9.7.3.1 Event Transport
  S-9.7.3.2 Datagram Transport TN-9.7.3.2 Datagram Transport
  9.7.4 SESSION LAYER  
  S-9.7.4.1 Configuration Description Information TN-9.7.4.1 Config Description Info
  S-9.7.4.2 Memory Configuration TN-9.7.4.2 Memory Configuration
  S-9.7.4.3 Simple Node Information TN-9.7.4.3 Simple Node Information
  S-9.7.4.4 Firmware Upgrade TN-9.7.4.4 Firmware Upgrade
  S-9.7.4.5 Broadcast Time Protocol TN-9.7.4.5 Broadcast Time Protocol
     

** Renumbered; ♦ Document(s) Under Revision - see Documents Under Revision page

Recommended Practices (RP)

 

Technical Notes (TN), Technical Information (TI) and Technical References (TR)

Technical Notes

Technical In formation

Technical References

NMRA Standards documents are copyrighted by NMRA under US and international copyright laws. They are made available by NMRA and are adopted for a wide variety of both public and private uses. These include both use, by reference, in laws and regulations, and use in private self-regulation, standardization, and the promotion of modeling, structural and engineering practices and methods. By making NMRA Standards documents available for use and adoption by public authorities and private users, NMRA does not waive any rights in copyright to the NMRA Standards documents. Permission to reproduce this copyrighted publication in whole or in part may be obtained upon written application to the headquarters of the NMRA.

Module Standards (MS)

Module Recommended Practices (MRP)

Updated 13-Dec-2024 commeents/corrections: tech-elec [at] nmra.org (subject: nmra.org%3A%20Standards%20and%20Recommended%20Practices%20Page)