FRA Widens Locomotive Wheel Size Limits for AC Motors
Published Date: 8/31/2026
Rule
Summary
This rule amends FRA's locomotive safety regulations to expand the maximum permitted variation in diameter for locomotive wheel sets using alternating current technology, in response to a Class I railroad's May 2019 petition for rulemaking and innovations in traction motor control.
Analyzed Economic Effects
3 provisions identified: 1 benefits, 0 costs, 2 mixed.
Larger wheel-diameter allowance for AC locomotives
The rule changes 49 CFR 229.73 to allow a 1.5-inch maximum diameter variation for powered axles on locomotives with alternating current (AC) traction motors that use individual truck or single-axle control technology. The rule keeps the existing limits for other situations: 3/4 inch within the same three-powered-axle truck (or up to 1 1/4 inches with shims) and 1 1/4 inches between different trucks; paragraph (b)(3) does not apply to non-powered axles in the middle of two powered axles.
Industry cost savings and reduced downtime
FRA expects the rule will produce cost savings for the railroad industry by increasing the allowable wheel-diameter threshold for certain AC locomotives, which should reduce locomotive out-of-service time and increase wheel life. FRA states it does not anticipate any costs from this final rule and that small entities choosing to use the flexibility will experience benefits.
Labor groups raise safety and ride-quality worries
Several labor organizations (BLET, IAM Union, SMART-TD, TTD) opposed raising the limit to 1 1/2 inches, saying it could increase derailment risk, bearing and gearbox stress, wheel slip, and rougher cab rides; some commenters noted manufacturers recommend less than 1 inch. FRA reviewed comments, relied on a GE technical analysis, and adopted the amendment despite those objections.
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Key Dates
Department and Agencies
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