Assessment of Service-Worn M-976 Trucks

Written by Alexander Keylin, Senior Engineer II; Russell Walker, Principal Investigator II; Chris Pinney, Senior Economist II; Tony Sultana, Principal Investigator I; and Ryan Alishio, Engineering Data Analyst, Transportation Technology Center, Inc.
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William Beecher photo

RAILWAY AGE, FEBRUARY 2020 ISSUE: Even though M-976 dynamic performance may degrade with age and mileage, they appear to be less likely to cause derailments than non-M-976 trucks of similar age.

In the years since Association of American Railroads’ (AAR) 2003 adoption of Specification M-976, hundreds of thousands of M-976-compliant trucks have been built. Some of them are now being taken out of service to be rebuilt under car owners’ planned maintenance programs. Transportation Technology Center, Inc. (TTCI) collaborated with several Class I railroads and a private car owner to identify a sample of service-worn trucks and perform measurements and tests to see how wear affects their dynamic performance.

Based on information from car owners, 26 railcars with gross rail loads (GRL) of 286,000 pounds were identified as candidates for this study. Based on their maintenance records, mileage and truckside wedge rise (a proxy indicator for overall truck wear), trucks from six of these railcars were selected for more detailed measurements. These truck samples included models by two manufacturers; their service mileage ranged from 525,000 to 886,000 miles.

The 12 trucks from the six selected railcars were disassembled and examined for signs of wear or damage, and had detailed measurements of their components taken. Although only one truck had its wedge rise exceed the prescribed limit, each of the 12 trucks had some of their components (such as the pedestal jaws, friction wedges or column wear plates) worn to prescribed limits. A majority of the 12 trucks had broken springs (which were replaced prior to testing). One truck had a cracked adapter pad, and two trucks were showing partial delamination of adapter pads (Figure 1).

Figure 1. (a) Worn center bowl liner; (b) cracked adapter pad;
(c) delaminating adapter pad. TTCI photos.

Four of the truck sets (eight trucks) were then tested under selected M-976 testing regimes. None of them met all of the M-976 acceptance criteria in twist and roll, pitch and bounce, and hunting test regimes. For the dynamic curving test, one truck set did not meet the acceptance criteria, and testing of two more truck sets was interrupted because of safety concerns (significant wheel unloading or poor steering—the latter likely caused by the condition of a center bowl liner).

In addition to testing worn M-976 trucks, TTCI analyzed the data from the Federal Railroad Administration (FRA) Accident/Incident Database and UMLER® system to determine if derailment rates due to truck-related causes (poor steering, defective snubbing, etc.) were different in cars equipped with M-976 trucks vs. non-M-976 trucks.

The data showed that from 2003 to 2017, 598,198 railcars with 286,000 pounds GRL were built—429,769 of them were equipped with M-976 trucks, and 168,429 with non-M-976 trucks. Within those car populations, four cars equipped with M-976 trucks were involved in truck-caused derailments (two on main line track, one on yard track and one on industry track), compared to 20 cars equipped with non-M-976 cars (seven derailments on main line track, 10 on yard track, one on a siding and two on industry track). The most common cause of truck-caused derailments in either group was improper truck rotation. A non-parametric survival analysis concluded that among the cars with a GRL of 286,000 pounds built between 2003 and 2017, cars with M-976 trucks had a statistically lower rate of truck-related derailments than cars with non-M-976 trucks.

This study shows that even though the dynamic performance of M-976 trucks may degrade with age and mileage, they appear to be less likely to cause derailments than non-M-976 trucks of similar age.

The M-976 fleet is still relatively young, and truck-caused derailments are rare. Therefore, this statistical analysis should be repeated in 3 to 5 years when more M-976 trucks acquire longer time in service and more data on derailments becomes available. In addition, it is recommended to test several M-976 trucks with wedge rise over the prescribed limit, should they become available. This testing could help assess whether the current prescribed limits for wedge rise are appropriate.

This research was performed as a part of the AAR’s Strategic Research Initiatives program. Complete results of this study can be found in previously published TTCI/AAR Technology Digests and research reports.


  • Keylin, A., Walker, R., Pinney, C., Sultana, T., and Alishio, R., “Assessment of Service-Worn, 110-ton M-976 Trucks,” Technology Digest TD-19-020, September 2019.
  • Keylin, A., Walker, R., and Tournay, H., “Condition and Performance Assessment of Four Service Worn 110-ton M-976 Approved Freight Trucks,” Report R-1020, AAR/TTCI, 2017.
  • Walker, R., Keylin, A., Pinney, C., and Alishio, R., “Condition and Performance Assessment of Eight Service-Worn, 110-ton, M-976 Approved Freight Trucks,” Report R-1029, AAR/TTCI, 2017.
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