California Air Resources Board

Locomotives are self‑propelled, rail‑mounted off‑road equipment used to push or pull trains. The combination of locomotive(s) pulling freight or passenger railcars forms a train. Higher horsepower line haul locomotives and lower horsepower switcher locomotives operate in California. A typical freight and passenger locomotive in the United States is powered by a diesel engine that drives an electrical generator or alternator. Locomotives and railyards are a source of harmful nitrogen oxides (NOx) emissions, particulate matter (PM 2.5) emissions, and greenhouse gas (GHG) emissions. Other sources of NOx and PM2.5 emissions in railyards include railcar movers, yard trucks, drayage trucks, and other types of cargo handling equipment. The California Air Resources Board (CARB) has developed and implemented a number of measures to understand and reduce locomotive and railyard emissions, including studies, regulations, enforceable agreements, and funding of clean technology.

On April 27, 2023, the In‑Use Locomotive Regulation (Regulation) was approved by the Board to help meet California’s public health, air quality, and climate goals. The Regulation would have achieved emission reductions from diesel‑powered locomotives and increased the use of zero‑emission (ZE) technology. CARB did not receive authorization for the Regulation from the United States Environmental Protection Agency (U.S. EPA) and was unable to enforce the Regulation. The Regulation’s presence in the California Code of Regulations could have caused confusion and uncertainty for California locomotive operators. Therefore, CARB repealed the Regulation in June 2025. CARB is still committed to reducing rail emissions and is working with communities and industry to do so.