About AAR Circular Letters
Circular letter summaries are produced monthly by Greenbrier Management Services Group.
AAR circular letters provide information of general applicability to the railroad industry, including proposed and final rules, revisions, standards and recommended practices, and calls for comments. These summaries provide the main points of circular letters issued in the prior month, as well as important implementation deadlines.
For access to the complete circular letter, please go to my.aar.org/circulars. Circular letters are a fee-based service provided by the AAR’s Transportation Technology Center Incorporated (TTCI). You may subscribe online at publications@aar.com or call 719.584.0538.
Implementation Circulars
C-14606
Implementation of Revisions to MSRP Section G, WHEELS AND AXLES, S-631, and S-632, and MSRP Section G-II, WHEEL AND AXLE MANUAL, RP-631, RP-633, and RP-634, Alternate Combination Gauge
Issued: August 29, 2025
Effective: August 29, 2025
The purpose of this circular letter is to notify the industry of revisions to MSRP Section G, WHEELS AND AXLES, S-631, and S-632, and MSRP Section G-II, WHEEL AND AXLE MANUAL, RP-631, RP-633, and RP-634, adding the alternate combination gauge and appropriate references.
On September 7, 2023, Circular Letter C-14207 was issued to solicit comments. Comments were received and considered by the WABL Committee, but determined no revisions were necessary.
Revised standards S-631, S-632, RP-631, RP-633, and RP-634, attached, are now implemented and effective immediately. This modification will be incorporated in the next issue of the Manual of Standards and Recommended Practices, Section G and Section G-II. Please update your library and be governed accordingly
C-14605
Implementation of Revised MSRP Section S-I, CASTING DETAILS, 2025
Issued: August 29, 2025
Effective: August 29, 2025
The Association of American Railroads Coupling System and Truck Castings Committee (CSTCC) is pleased to announce a new edition of the Manual of Standards and Recommended Practices, Section S-I, CASTING DETAILS.
Since the last printing issued in October 2021, the CSTCC has issued revisions through several Circular Letters. These Circular Letters are listed in the attached document. The revised MSRP Section S-I is now available in both printed and electronic versions. Current pricing is available online at www.aarpublications.com where the manual can be purchased.
All previously implemented standards and specifications are included in this print of Section S-I. Please update your library and be governed accordingly.
C-14607
IMPLEMENTED AAR Manual of Standards and Recommended Practices (MSRP) Section K-IV Office Architecture and Railroad Electronics Messaging, Standard S-9354.V3.0
Issued: August 29, 2025
Effective: August 29, 2025
S-9354.V3.0 specifies the Edge Message Protocol (EMP) message envelope. It describes the fields of the envelope, their semantics, and commentary about their use. Sufficient detail for implementation is provided. S-9354.V3.0 was Adopted in 2010 and Last Revised in 2025 and is attached to the circular in PDF format.
S-9354.V3.0 will be incorporated into the next issue of the MSRP Section K-IV Office Architecture and Railroad Electronics Messaging. In the interim, please insert the Circular in your copy of the MSRP Section K-IV and be governed accordingly.
There is external content to the document, Appendix B – EMP Message ID’s V1.0. This content has been updated as well. The version number of the external content may not be the same as this document due to update frequency. This content is stored separately from the document and will be published in an additional circular letter.
C-14608
IMPLEMENTED AAR Manual of Standards and Recommended Practices (MSRP) Section K-IV Office Architecture and Railroad Electronics Messaging, Standard S-9354 – Appendix B V1.0
Issued: August 29, 2025
Effective: August 29, 2025
S-9354 – Appendix B.V1.0 – EMP Message ID’s is external content to S-9354.V3.0 Edge Message Protocol. Appendix B is a spreadsheet containing all the current EMP Message ID assignments.
The update frequency of the AAR document and the content of this appendix is not the same, therefore the information in the appendix is stored separately from the document. Nevertheless, this information shall be considered part of Standard S-9354.
S-9354 – Appendix B.V1.0 is attached to the Circular in PDF format. S-9354’s latest version and all external content will be incorporated into the next issue of the MSRP Section K-IV Office Architecture and Railroad Electronics Messaging. In the interim, please insert the circular in your copy of the MSRP Section K-IV and be governed accordingly.
Call for Comment
C-14599
Revisions to MSRP Section C-I, CAR CONSTRUCTION FUNDAMENTALS AND DETAILS, Standard S-234, and Section C-II M-1001 DESIGN, FABRICATION, AND CONSTRUCTION OF FREIGHT CARS, regarding jacking and lifting requirements
Issued: August 1, 2025
Effective: 30 days for comment.
The purpose of this circular letter is to solicit comments for Revisions to MSRP Section C-I, CAR CONSTRUCTION FUNDAMENTALS AND DETAILS, Standard S-234, and Section C-II M-1001 DESIGN, FABRICATION, AND CONSTRUCTION OF FREIGHT CARS, Chapters 2, 4, and 6, regarding jacking and lifting requirements.
The intent of this effort is to improve the description of jacking and lifting requirements in response to issues encountered during handling of recently built cars. See Circular for full details on the proposed drafts.
C-14601
Revisions to MSRP Section C-I, CAR CONSTRUCTION FUNDAMENTALS AND DETAILS, Standard S-2044, Safety Appliance Requirements for Freight Cars, Editorial Updates to Appendices F1 and F2
Issued: August 8, 2025
Effective: 30 days for comment.
The purpose of this circular letter is to solicit comments for editorial revisions to MSRP Section C, CAR CONSTRUCTION FUNDAMENTALS AND DETAILS, Standard S-2044, Safety Appliance Requirements for Freight Cars.
The drawings in Appendix F1 (Safety Appliances for Open-Top Hopper Cars and High-Side Gondola Cars) and Appendix F2 (Safety Appliances for Low-Side and Hinged-End Gondola Cars and Bulkhead Flatcars with Low Bulkheads) were reformatted to better illustrate how to measure clearance for the highest handhold at the BL corner and at the AR corner. Proposed revisions are highlighted with change bars in the draft of Appendices F1 and F2 attached to the circular.
C-14602
Revisions to MSRP Section H-II, ROLLER BEARING MANUAL, Standards S-735, S-736, S-737, S-738, and S-740, Spall and Gouge Criteria, Polymer Cages, and Temperature Measurements
Issued: August 14, 2025
Effective: 30 days for comment.
The purpose of this circular letter is to solicit comments for proposed revisions to MSRP Section H-II, ROLLER BEARING MANUAL, Standards S-735, S-736, S-737, S-738, and S-740. See the draft attached to the Circular for full details. The proposed revisions to S-735, S-736, S-737, S-738, and S-740 have costs associated with implementation, and those costs are below the thresholds prescribed in AAR Standard S-050.
C-14604
DRAFT S-9076.V2.0 ITC Systems Management Interoperable Gateway Requirements, in support of AAR’s MSRP – Section K-I Railway Electronics Systems Architecture and Concept of Operations
Issued: August 27, 2025
Effective: 10 days for comment.
The Positive Train Control Interoperability Committee (PTCIC) of the Association of the American Railroads (AAR) is publishing DRAFT S-9076.V2.0 ITC Systems Management Interoperable Gateway Requirements for review and comment. This document describes the ITCSM Gateway requirements for processing interoperable ISMP messages.
In order to provide time for review and comments, this document will have a 10-day comment period. The time will begin with the posted date.
Notice of Circulars
C-14600
MSRP K-III, RP-9203 AEI Reader Equipment Requirements
Issued: August 1, 2025
Effective: August 1, 2025
This circular is being released to the industry to advise of minor editorial and reference updates made to the subject recommended practice.
As noted in prior circulars C-14560 and C-14584, AAR’s AEI Committee has pursued a multi-stage plan to evaluate and modernize the governing standards within its purview for ease of use, accuracy, and to ensure quality, up to date content.
Modifications will be incorporated in the next issue of the Manual of Standards and Recommended Practices, Section K-III. Please update your standards reference library and be governed accordingly.
C-14603
Relocation of MSRP S-I, CASTING DETAILS, Specification M-975, Span Bolsters, to MSRP C-I CAR CONTRUCTION FUNDAMENTALS AND DETAILS
Issued: August 15, 2025
Effective: August 15, 2025
The purpose of this circular letter is to inform MSRP users of the relocation of MSRP S-I, CASTING DETAILS, Specification M-975, Span Bolsters, to MSRP C-I CAR CONTRUCTION FUNDAMENTALS AND DETAILS. The Equipment Engineering Committee and Coupling System and Truck Castings Committee have agreed to relocate M-975 to align with committee responsibilities.
Specification M-975 will be incorporated in the next issue of the MSRP, Section C-I. In the online MSRP system M-975 will be available under Sections S-I and C-I until the next issue of MSRP S-I, at which time it will be removed from MSRP S-I. The next issue of MSRP S is expected within the next few weeks. Please update your library and be governed accordingly.
AAR Hazmat & Tank Car Committee meetings
October 13–16, 2025 | Addison, TX
The Greenbrier Companies is a proud exhibitor and sponsor.
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