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.  

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 General Overview - (4/2021)
  • 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-3.1 Trackwork - Proto Scales (1/2006)
  • TN-1.1.1 for S-3.1 PROTO:SCALE and FINE SCALE GUIDE - (10/31/2003)
  • TN 1.1.2 for S-3.1 PROTO:SCALE AND FIND SCALE GUIDE - (11/22/2005)
• S-3.2 Trackwork - Standard Scales - (5/8/2010)
• S-3.3 Trackwork - Deep Flanges for Guarded Track - (2/24/2010)
• S-4.1 Wheels - Proto Scales - (1/2006)
  • TN-1.1.1 for S-4.1 PROTO:SCALE and FINE SCALE GUIDE - (10/31/2003)
  • TN 1.1.2 for S-4.1 PROTO:SCALE AND FIND SCALE GUIDE - (11/22/2005)
• S-4.2 Wheels - Standard Scales - (01/2019)
• S-4.3 Wheels - Large Scale Standards, Wheels with Deep Flanges - (2/2010)
• S-5 TRACTION Power Collection - (8/1982)
  • RP-5 Electric Traction - General - with Proposal for Member Comment - (2/1982)
    • RP-5.1 Electric Traction - Trolley Wire Frogs - (2/1982)
    • RP-5.2 Electric Traction - Curvature & Drawbars - (2/1982)
• S-9 ELECTRICAL - (1984)
  • RP-9 Electrical - General - (5/1973)
  • TN-9 Wiring for DC and DCC  (12/30/2023 Revised)
    • 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  (12/1/2020)
          • 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 and Next 18S Decoder Interface  (12/6/2020)
          • TN-9.1.1.5 Next 18 and 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 DCC Communications Standard - (2004)
        • S-9.2.1 DCC Extended Packet Formats - (5/15/2022) Under revision see DCC section for DRAFT copy
        • S-9.2.1.1 DCC Advanced Extended Packet Formats - (5/15/2022)
          • TN-9.2.1.1 DCC Advanced Extended Packet Formats - (5/15/2022)
        • S-9.2.2 DCC Configuration Variables - (2012) Under revision see DCC section for DRAFT copy
        • S-9.2.2 Appendix A, DCC Manufacturer ID codes - (revised March 2021)
        • 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 Standard
          • S-9.3.1 (discontinued)
          • S-9.3.2 DCC Basic Decoder Transmission - (updated 12/20/2012) Under Revision see DCC section for DRAFT copy
        • S-9.7.x  Layout Command Control (LCC) - the LCC page(s) located here.

Recommended Practices (RP)

• RP-1 Abbreviations - (4/2021)
• RP-2 Standards Gauges - (01/15/2024) [Includes data from former RP-8 Three Point Track Gauges(OBSOLETE)]
  • Note: For Help with NMRA Gauge letter definitions and the AAR Plate diagrams on which it's based see RP-2.
  • Note: The NMRA does not sell a Standards gauge for S scale, (standard gauge). Modellers looking for an S scale gauge should contact the National Association of S Gaugers at:http://www.nasg.org. The NMRA does have the Sn3 gauge available in the NMRA Store.
    • TN-2 - Standards Gauges Instruction Sheet (08/19/2023)
• RP-5 Electric Traction - General - with Proposale for Member Comment - (2/1982)
  
  • RP-5.1 Electric Traction - Trolley Wire Frogs - (2/1982)
  • RP-5.2 Electric Traction - Curvature & Drawbars - (2/1982)
• RP-7 Track Centers and Obstacle Clearances - (7/2017) [Includes material formerly found under S-6, S-7 and S-8]
  • TN-7 Track Center and Obstacle Clearance Methodology (7/2017) 
  • ​​RP-7.1 Tangent Track Centers and Clearance Diagrams (1/2019) 
  • RP-7.2 Curved Track Centers (7/2017) 
  • RP-7.3 Curved Track Obstacle Clearances (7/2017) 
  • RP-7.4 Interurban Track Centers and Obstacle Clearances (7/2017) 
  • RP-7.5 Modifying Tangent Clearance Diagrams for Curved Tracks (7/2017) 
  • RP-7.6 Using the NMRA Curved Track Center and Obstacle Clearance Assistant (7/2017) 
  • ​Curved Track Center and Obstacle Clearance Assistant (tool link)
• RP-9 Electrical - General - (5/1973)
• RP-10 Trackwork - General - (7/2012)
• RP-11 Curvature and Rolling Stock - (3/2018) 
  • ​TN-11 Curvature and Rolling Stock - (4/2017)
• RP-12 Turnouts - General - (2/2015)
  • RP-12.x (zip file contains 12.1 to 12.54) (2/2015)
  • TN-12 Engineering Analysis and Geometric Design of Model Railroad Turnouts
  • Excel Spreadsheet Generalized Turnout Design, Generates Model Turnout Dimensions
• RP-13
  • RP-13.5 Guard Rail & Frog Relationship - (8/1981) 
  • RP-13.6 Guard Rails - (8/1981)
  • RP-13.7 Frog & Wing Rails - (8/1981)
  • RP-13.8 Flangeway Flares - (8/1981)
• RP-15
  • RP-15.1 Rail - (1/1990)
• RP-20
  • RP-20.1 Car Weight - (1/1990)
• RP-21 Couplers - (7/9/2010)
  • RP-21.1 Coupler Contour - (8/1957)
  • RP-21.2 Solid Couplers - (2/1960)
• RP-22 Coupler Pocket - HO Scale - (8/1958)
• RP-23 Bolsters - (12/6/2011)
• RP-24 Trucks - (8/1963)
  • RP-24.1 Journals - (7/1982)
  • RP-24.2 Wheelsets - (8/1963)
  • RP-24.3 Axles - (7/1982)
• RP-25 Wheel Contour (7/2009)
• RP-37 Locomotive/Tender -Drawbar & Pin Relationship - (3/1997)
  • RP-37.1 Locomotive/Tender -Drawbar & Pin Dimensions - (3/1997)

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

Technical Notes

• TN-1.1.1 for S-3.1 PROTO:SCALE and FINE SCALE GUIDE - (10/31/2003)
• TN 1.1.2 for S-3.1 PROTO:SCALE AND FIND SCALE GUIDE - (11/22/2005)
• TN-2 - Standards Gauges (01/15/2024)
• TN-7 Track Center and Obstacle Clearance Methodology (7/2017)
• TN-9 Wiring for DC and DCC  (12/30/2023 Revised)
• TN-9.1.1 NMRA DCC Decoder Test User Manual (8/12/2015)
• TN-9.1.2 Sender Board Theory of Operation (8/12/2015)
• TN-9.1.3 Sender V3 Getting Started Guide (8/12/2015)
• TN-9.1.1.3 21 MTC Decoder Interface (12/1/2020)
• TN-9.1.1.4 PluX Decoder Interface (6/12/2021)
• TN-9.1.1.5 Next 18 and Next 18S  Decoder Interface (9/10/2020)​
• TN-9.1.2 Power Station Interface (6/9/2021)
• TN-9.2.1.1 DCC Advanced Extended Packet Formats - (5/15/2022)
• TN-11 Curvature and Rolling Stock - (4/2017)
• TN-12 Engineering Analysis and Geometric Design of Model Railroad Turnouts

Technical In formation

• TI-9.1.1 Sources for Connectors for DCC Decoders - (11/19/2021)
• TI-9.2.3 Serial User Standard Interface for DCC - (5/2003)

Technical References

• TR-21, LS Coupler LARGE SCALE COUPLERS - (6/15/2010)
• TR-1-2020 Standards & Conformance Department, Standards Document and Format Instructions (2/7/2021)
• TR-3-2020 NMRA Standards and Conformance Department Participation (3/3/2021)
  • Frequently Asked Questions about TR-3-2020 dtd 5 March 2021
    • These questions should answer who signs which attachments and why
• TR-1-2023 Inspection Process and Procedures (12/30/2323)

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)

• Introduction to Modules (Read for construction specifications)
• MS-1.0 STANDARD GAUGES (1/1990)
• MS-1.1 NARROW GAUGE   (1/1990)
• MS-1.2 TRACTION/OVERHEAD STANDARDS   (1/1990)
• MS-1.3 ELECTRICAL, ALL SCALES  (1/1990)

Module Recommended Practices (MRP)

• MRP-1.0 STANDARD GAUGES - (1/1990)
• MRP-1.1 NARROW GAUGE - (1/1990)
• MRP-1.2 TRACTION/OVERHEAD - (1/1990)
• MRP-1.2A TRACTION/OVERHEAD - (1/1990)
• MPR-1.3 ELECTRICAL - (1/1990)
• MRP-1.4 TRANSITION MODULE, ALL SCALES - (1/1990)

Updated 06-February-2024