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
CPC-1431
Implementation of Appendix E, AAR- MSRP-CIII (M-1002)
Issued: July 3, 2025
Effective: July 3, 2025
The Tank Car Committee authorized changes to the language in Appendix E, paragraph 8.4, 8.4.1 and 8.4.5 of the MSRP-CIII, Specifications of Tank Cars (M-1002) regarding requirements for bottom fittings protections that apply only to external ball valves.
Revision is hereby implemented and in effect as of the publication date of this circular.
C-14591
Revisions to MSRP Section H-II, ROLLER BEARING MANUAL, S-736, S-737, S-738, and S-740, Inspection Probe, and Backing Ring Fillet Gauge
Issued: July 13, 2025
Effective: July 13, 2025
The purpose of this circular letter is to notify the industry of revisions to MSRP Section H-II, ROLLER BEARING MANUAL, Standards S-736, S-737, S-738, and S-740, adding instructions on how to inspect the roller assembly and cup using the inspection probe, and adding a new gauge for inspecting the backing ring fillet.
On April 15, 2025, Circular Letter C-14535 was issued to solicit comments. Comments were received and considered by the WABL Committee. Revised standards are now implemented and effective immediately. This modification will be incorporated in the next issue of the Manual of Standards and Recommended Practices, Section H-II. Please update your library and be governed accordingly
C-14593
Revisions to MSRP Section S-II, TRUCK DETAILS AND CASTING CODES, Standard S-378, Side Frame, Cast Steel–Making and Mating, regarding proper button casting and removal
Issued: July 13, 2025
Effective: July 13, 2025
The purpose of this circular letter is to notify the industry of revisions to Section S-II, TRUCK DETAILS AND CASTING CODES, Standard S-378, Side Frame, Cast Steel–Making and Mating.
On May 29, 2025, Circular Letter C-14578 was issued to solicit comments. No comments relating to the proposed changes were received. The revised standard is now implemented and effective immediately. This modification will be incorporated in the next issue of the Manual of Standards and Recommended Practices, Section S-II. Please update your library and be governed accordingly.
C-14592
Revisions to MSRP Section S-I, CASTING DETAILS, Specification M-203, Truck Side Frames, Cast Steel – Design and Testing, regarding updates to facility, equipment, and test approval
Issued: July 16, 2025
Effective: July 16, 2025
The purpose of this circular letter is to notify the industry of revisions to Section S-I, CASTING DETAILS, Specification M-203, Truck Side Frames, Cast Steel – Design and Testing.
On February 7, 2025, Circular Letter C-14507 was issued to solicit comments. Changes were proposed to clarify requirements for static and dynamic test observation and facility/equipment process and requirements. No comments were received.
The revised specification is now implemented and effective immediately. This modification will be incorporated in the next issue of the Manual of Standards and Recommended Practices, Section S-I. Please update your library and be governed accordingly.
C-14596
AAR Manual of Standards and Recommended Practices (MSRP) Section K-II Locomotive Electronics and Train Consists System Architecture, Standard S-9103.V1.1
Issued: July 23, 2025
Effective: July 23, 2025
S-9103.V1.1 specifies an industry-standard ICD that defines high precision navigation messages for PNM’s on locomotives. This standard applies to all PNM devices ordered after January 1, 2021.
S-9103.V1.1 was Adopted in 2021 and Last Revised in 2025 and is attached to this circular in PDF format. S-9103.V1.1 will be incorporated into the next issue of the MSRP Section K-II Locomotive Electronics and Train Consists System Architecture. In the interim, please insert the Circular in your copy of the MSRP Section K-II and be governed accordingly.
Call for Comment
C-14588
Revisions to MSRP Section B FREIGHT CAR DRAFT COMPONENTS, Specification M-921H, Cushioning Devices, End-Of-Car, General Requirements, addition of Gas Separation Requirements for Hydraulic Cushioning Devices
Issued: July 2, 2025
Effective: 30 days for comment.
The purpose of this circular letter is to solicit comments on revisions proposed to Section B FREIGHT CAR DRAFT COMPONENTS, Specification M-921H, Cushioning Devices, End-Of-Car, General Requirements. Paragraphs 6.3, 6.3.1, and 6.3.2 have been added to make gas separation tubes required for all new cushion units and certain reconditioned units beginning January 1, 2026.
CPC-1430
Solicitation for comment of Appendix A, AAR- MSRP-CIII (M-1002)
Issued: July 3, 2025
Effective: 30 days for comment.
The Tank Car Committee authorized the replacement of table A.2 in Appendix A with D.6 from Appendix D in the MSRP-CIII, Specifications of Tank Cars (M-1002). This table now includes serial numbers marked at the time of initial manufacturing or assembling and AAR component tracking effective dates.
The committee additionally approved the requirement of “original valve date of manufacture” as a required markings for Pressure Relief Valve (6.2.1), Combination Devices (6.2.2), Rupture Disc Devices (6.2.3.1), Vacuum Relief Valve (6.2.4), Liquid and Vapor Valves (6.3.1), Pressure Regulator Valves (6.3.2), Excess Flow Valves (6.3.3), Liquid Level Control and Gauging Device (6.4.1, 6.4.1.1).
Revisions are being issued for comment.
C-14590
DRAFT S-9070 External Content V0.0 ITC Human Machine Interface Standard, in support of AAR’s MSRP – Section K-I Railway Electronics Systems Architecture and Concepts of Operations
Issued: July 13, 2025
Effective: 7 days for comment.
The Positive Train Control Interoperable Committee (PTCIC) of the Association of American Railroads (AAR) is publishing DRAFT S-9070 External Content V0.0 ITC Human Machine Interface Standard for review and comment.
External content V0.0 to ITC Human Machine Interface Standard, S-9070.V2.0 defines the human-machine interface standards for the initial version of OPA software that supports Positive Train Control Office – Locomotive Segment – Interface Control Document (ICD), S-9361.V3.1, which specifies Interface Version 4 (IV4).
C-14589
DRAFT S-9051 External Content V0.0 ITC Onboard PTC Application (OPA) Requirements, in support of AAR’s MSRP – Section K-I Railway Electronics Systems Architecture and Concepts of Operations
Issued: July 13, 2025
Effective: 7 days for comment.
The Positive Train Control Interoperable Committee (PTCIC) of the Association of American Railroads (AAR) is publishing DRAFT S-9051 External Content V0.0 ITC Onboard PTC Application (OPA) Requirements for review and comment.
External content V0.0 to ITC Onboard PTC Application (OPA) Requirements, S-9051.V1.0, specifies the OPA requirements for interoperable PTC for the initial version of OPA software that supports Positive Train Control Office – Locomotive Segment – Interface Control Document (ICD), S-9361.V3.1, which specifies Interface Version 4 (IV4).
C-14595
Revisions to MSRP Section C-I, CAR CONSTRUCTION FUNDAMENTALS AND DETAILS, Proposed new Standard S-2058 Non-Hydraulic Energy Management Systems (EMS) with Buff Travel of 6″ or more
Issued: July 21, 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, with addition of proposed new Standard S-2058 Non-Hydraulic Energy Management Systems (EMS) with Buff Travel of 6″ or more. A draft of S-2058 is attached to the circular.
C-14594
Revisions to MSRP Section AS ADMINISTRATIVE STANDARDS SUPPLEMENT, Standard S-046 Definitions of Terms for the AAR Manual of Standards and Recommended Practices, regarding updating of definitions and addition of context meaning
Issued: July 21, 2025
Effective: 30 days for comment.
The purpose of this circular letter is to solicit comments for revisions proposed to Section AS Standard S-046 Definitions of Terms for the AAR Manual of Standards and Recommended Practices (MSRP), regarding updating of definitions and addition of context meanings commonly used throughout the MSRP.
A draft of the revised standard is attached to the circular. The entire document is highlighted with change bars as it has been completely reworked.
C-14597
Proposed New Section added to MSRP, Section M – LOCOMOTIVES AND LOCOMOTIVE INTERCHANGE EQUIPMENT: Standard, S-5018 – DYNAMIC BRAKING CONTROL
Issued: July 25, 2025
Effective: 30 days for comment.
The purpose of this circular letter is to solicit comments for revisions proposed to Manual of Standards and Recommended Practices (MSRP) Section M, standard S-5018 – DYNAMIC BRAKING CONTROL. Per a Railway Electronics Standards Committee (RESC) request to the Locomotive Committee (LC), a task force was created and convened to identify dynamic brake warning criteria and standardize within S-5018 DYNAMIC BRAKING CONTROL. The LC-approved task force recommendations have been added as a new section (4.0 DYNAMIC BRAKE WARNING) within the draft standard.
Notice of Circulars
C-14598
Fall 2025 AAR Technical Committee Meetings
Issued: July 31, 2025
Effective: July 31, 2025
Details for AAR meetings that have been planned for the second half of 2025 have been posted.
AAR Meeting Announcements can also be found online on the MxV Rail Technical Services web page: http://www.aar.com/standards/AARMeetingAnnouncements.html
Information for individual Technical Committee Meetings that will be held in Colorado Springs in August can be found online at https://cvent.me/5v5YeK
Greenbrier Proudly Introduces: GBX Training Tank Car™
This state-of-the-art tank car training platform is designed to enhance rail safety and provide hands-on learning for railcar operators, maintenance crews and safety professionals.
Need Railcar Maintenance Services?
Our repair facilities are ready and equipped to handle everything your fleet needs.
Greenbrier’s Regulatory Services Group
Peace of mind for your business.
Led by Diana Aylward, Greenbrier’s Regulatory Management Group provides regulatory, engineering, process consulting and advocacy support to the railcar repair industry and the petrochemical rail shipper community.
Our core services include:
- DOT Compliance Assessments (Corporate and Plant)
- DOT Hazardous Materials Training (Corporate and Plant)
- Federal Claims Mitigation Support (e.g., FRA and PHMSA)
- Regulatory Counsel and Policy Advocacy
- Railcar Repair Management (Design Review, Drawings, Engineering and AAR Approval Administration)
- Tank Car Reliability & Maintenance Programs (per 49 CFR Subpart F of Part 180)
- Tank Car Maintenance Data Collection & Performance Analysis
For inquiries about these summaries or any of our services, please reach out to: regulatoryservices@gbrx.com
About Greenbrier Management Services (GMS)
Greenbrier Management Services (GMS) is North America’s most comprehensive railcar management solutions provider. GMS manages nearly 400,000 railcars and has invested over 20 years in developing technology, processes, and people to provide the best railcar management products and services in the industry. GMS is powered by proprietary software and the best subject matter experts in the railcar business.
COMPREHENSIVE. INTEGRATED. VALUE.
Combined, these two vitally important capabilities are able to deliver world-class solutions to every type of customer, from Class I railroads, lessors and industrial shippers to financial investors. GMS brings broad expertise to cover nearly any railcar management need that customers may have and its integrated solutions and reporting can be customized for any customer function or management need.