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U.S. Department of Transportation U.S. Department of Transportation Icon United States Department of Transportation United States Department of Transportation

Interpretation Response #21-0054

Below is the interpretation response detail and a list of regulations sections applicable to this response.

Interpretation Response Details

Response Publish Date:

Company Name: TCIX Rail

Individual Name: Keenan Moore

Location State: TX Country: US

View the Interpretation Document

Response text:

August 25, 2021

Keenan Moore
Fleet Management Specialist
911 NW Loop 281 Suite 316
Longview, TX  75604

Reference No. 21-0054

Dear Mr. Moore:

This letter is in response to your March 26, 2021, email requesting clarification of the Hazardous Materials Regulations (HMR; 49 CFR Parts 171-180) applicable to minimum plate thickness requirements for DOT-111 tank cars. Specifically, you ask what minimum plate thickness should be used when determining the permitted thickness reduction adjustments in accordance with § 180.509 and the Association of American Railroads (AAR) Manual of Standards and Recommended Practices (MSRP) Section C-III Specifications for Tank Cars (M-1002).

You state that you use the minimum plate thickness provided in § 179.201-1 for DOT-111 cars; however, you note that the MSRP has additional thickness requirements listed for tank cars constructed after 2004, based on how the tank car is equipped (e.g., shell material type, jacket, head shield, etc.).

We have paraphrased and answered your questions as follows:

Q1. You ask what minimum plate thickness should be used when calculating permitted thickness reductions in accordance with the thickness test requirements provided in § 180.509(f).

A1. For compliance with the HMR, and for tank cars that do not exceed 263,000 pounds gross weight on rail, the thickness values in § 179.201-1 of the HMR must be used to calculate the required minimum tank shell or head thickness, or value "B," as provided in § 180.509(f). As authorized by the Federal Railroad Administration (FRA), tank cars that have a gross weight on rail up to 286,000 pounds must adhere to the requirements specified in the approval notice titled "Operating Certain Railroad Tank Cars in Excess of 263,000 Pounds Gross Rail Load" (76 FR 4250). Note that the AAR may have additional minimum thickness requirements for the use of interchange railroads. You may use the contact information provided below to confirm AAR’s tank car requirements.

Q2. In the event that the specifications provided in Chapter 2 of M-1002 must be used as the "required minimum tank shell or head thickness after forming," you ask whether PHMSA would develop new specification classes that include those additional requirements (e.g., shell material type, jacket, head shield)—similar to what is provided in the specification table provided in § 179.201-1—or if those specifications would become grandfathered requirements that would result in noncompliant cars being phased out as they reach their 50-year end of life period.

A2. The answer is no. Except for tank cars containing poisonous-by-inhalation material, the requirements for a tank car exceeding 263,000 pounds are currently provided in § 179.13(a). No new specification is needed, as the tank cars built to AAR S-286 meet the DOT specification minimum thickness requirement.

Q3. You ask whether there would be a difference in the interpretation of the minimum thickness requirements between PHMSA and AAR that might result in noncompliance findings from Bureau of Explosives/ Transportation Technology Center, Inc. (BOE/TTCI) auditors and the FRA to various parties, depending on how they interpret these requirements.

A3. A PHMSA Letter of Interpretation—such as this letter—is intended to clarify the HMR and is not used to determine compliance with the regulations. Noncompliance findings cannot be determined within the scope of a letter and are not established for tank car specification compliance by this Office. The FRA Tank Car Team, Motive Power and Equipment (MP&E) Division and AAR's BOE/TTCI auditors should be contacted regarding their individual enforcement policies for a nonconforming tank car. The FRA Tank Car Team can be reached by email at, and a list of AAR points of contact is available at

I hope this information is helpful. Please contact us if we can be of further assistance.


Dirk Der Kinderen
Chief, Standards Development Branch
Standards and Rulemaking Division

179.13(a), 179.201-1, 180.509(f)

Regulation Sections

Section Subject
179.13 Tank car capacity and gross weight limitation
179.201-1 Individual specification requirements
180.509 Requirements for inspection and test of specification tank cars