Moving Beyond Zombie Engines: New Regulations Driving Cleaner Locomotion

February 23, 2024 by Paige Kendrick

Image of Baldwin Steam Locomotive 69 on the White Pass and Yukon Route; Photo Credit: Nils Öberg, CC BY-SA 3.0 via Wikimedia Commons (

In the hope to leave Zombie Engines in the past, the EPA recently promulgated a new final rule on Greenhouse Gas Emissions Standards for Heavy-Duty Vehicles. In this GELR blog post, Paige Kendrick analyzes the recent changes regarding federal preemption of non-new locomotives and locomotive engines as well as steps being taken by California to reduce locomotive emissions.

In the United States, the largest source of greenhouse gas (GHG) emissions is the transportation sector, contributing 38% of all fossil fuel combustion emissions as a result of tailpipe emissions.[1] According to an Environmental Protection Agency (“EPA” or “Agency”) Inventory of GHG emissions, “the fossil fuel combustion transportation subsector was the largest emitter of CO2 in 2021 followed by the electric power generation subsector.”[2] Within the transportation sector, trains and light rail, particularly in the freight sector, are some of the largest and most archaic polluters due to their use of “zombie” engines—old remanufactured high-pollution diesel engines—that are allowed to remain in service despite their poor environmental standards and harmful emissions. Part of this issue stems from the fact that the EPA has not updated pollution rules for locomotives since the late 2000s.[3] However, the Agency did recently publish a proposed rule for heavy-duty vehicles last April.[4]

In April 2023, the EPA published a proposed rule, Greenhouse Gas Emissions Standards for Heavy-Duty Vehicles — Phase 3, which focused primarily on heavy-duty highway vehicles.[5] A few months ago, on November 8, 2023, the EPA published a final rule from the sections of the proposal related to trains in the Federal Register. The final rule, “Preemption of State and local regulation of locomotives and engines used in locomotives,” went into effect on December 8, 2023, and:

implements a policy change to no longer categorically preempt certain State regulations of non-new locomotives and engines, aligning with the plain text of the Clean Air Act (CAA), and better achieving the legislative intent of providing for exclusive Federal regulation of new locomotives and new locomotive engines while preserving the ability of California and other States to adopt and enforce certain State standards regulating non-new locomotives and engines.[6]

Prior to this rule, the EPA asserted broad preemption over how state and local governments could control locomotives and locomotive engines, with a carve-out for “California to request authorization for other State requirements on non-new locomotives and [their] engines.”[7][8] This newly promulgated rule aims to reduce federal preemption on certain locomotives and locomotive engines to enable states—mainly California, with other states being allowed to follow suit—to adopt and enforce their own emission regulations.


Within the statutory scheme of federal preemption over emission regulations for locomotives and locomotive engines stemming from § 209 of the Clean Air Act, California serves as a state-level laboratory for innovation.[9] This notion of “states as laboratories” is a fundamental basis of federalism. While California appears to have the ability to regulate within the narrow parameters set forth in the CAA, the EPA’s final rule regarding locomotives in 1998 placed a “categorical bar on certain controls whether applicable to new or other locomotives and engines” that may have precluded California and other states from testing innovative programs to address locomotive pollution.[10] The rule the EPA recently finalized steps back from this overarching preemption and “ensures that such programs for non-new locomotives and engines may be authorized so long as they meet the statutory authorization criteria and in turn yield benefits for public health and the environment.”[11] Responding to those who did not think the EPA provided a compelling policy reason for this change, the EPA stated that the Agency believes this final rule better aligns with the plain text of the CAA.[12] The EPA views its changes as being supported by the record and better achieving the “legislative intent of providing for exclusive Federal regulation of new locomotives and new locomotive engines” while still allowing California the ability to obtain a waiver for new regulations on non-new locomotives and engines.[13]

This regulation applies only to non-new locomotives and locomotive engines, as the EPA is not retracting its regulatory power over new locomotives and new engines.[14] The EPA made this regulatory choice for a number of reasons, primarily because the EPA does not want to allow California and the states that may choose to follow its lead, to be able to impact the design and manufacturing of new locomotives and locomotive engines.[15] In doing this, the EPA is able to insulate manufacturers from significant regulatory changes that could damage the industry. Any significant state-to-state variance for new locomotives has the potential to create an unmanageable patchwork system of regulations that would stymie the production of locomotives, grinding innovation and productivity to a halt.

Towards the end of the final rule, the EPA notes that while locomotive emissions are an environmental justice concern, and one that they take seriously, the current action does not address environmental justice factors. This is because the final rule addresses statutory text regarding federal preemption and not human health or environmental conditions.[16] Since the rule does not speak to human health or the environment, it “cannot be evaluated with respect to potentially disproportionate and adverse effects on communities with environmental justice concerns.”[17] This does not mean that the final rule will not ultimately have an impact on emission reduction from non-new locomotives and their engines and the communities along high-traffic rail lines with EJ concerns, but rather that this rule is not intended to make such changes. What the final rule does change is the ways in which states and local governments can regulate non-new locomotives, known for their infamous zombie engines, allowing them to better protect and address the concerns of their local environmental justice communities.


In April of 2023, California passed the first emission rules for trains, ambitiously “limiting rail pollution to aggressively cut greenhouse gas emissions” and other air toxics.[18] Starting in 2030, the new regulation—“In-Use Locomotive Regulation” (California Rule)—will ban all non-exempt locomotives “with an Original Engine Build Date” prior to 2007, as well as “any Locomotive that is 23 years or older” upon each subsequent year.[19] The Original Engine Build Date has some degree of flexibility as it can “be based on the first year the Locomotive Engine was Remanufactured or Repowered to a Cleaner Locomotive.”[20] By preventing unnecessary rigidity, the California Rule incentivizes Locomotive Operators[21] to start upgrading their trains and engines to cleaner variations sooner rather than later. This allows operators to spread out the cost and “participate in incentive programs run by the state to ease the cost of transitioning to zero-emissions locomotives.”[22]

Additional regulations in the California Rule that are expected to instigate significant emission reductions include “Idling Requirements.” These requirements will compel Locomotive Operators to “ensure an AESS [Automatic Engine Stop/Start] equipped Locomotive Engine is shut off no more than 30 minutes after the Locomotive becomes stationary” with limited exceptions.[23] Locomotive engines are left idling in several conditions ranging from obvious track delays due to car crossings to the more surprising fact that switchyard locomotives usually keep their diesel engines running when the train is not moving.[24] This switchyard idling “can amount to several thousand hours of idling per locomotive each year.”[25] Idling locomotives emit a wide array of air toxics, such as PM10, PM2.5, NOx, and GHGs, but do so in a single location, causing higher concentrations of exhaust fumes and other hazardous substances in those areas. This is particularly relevant regarding train crossings and switchyards, as the continuous running of diesel engines occurs at a permanent location, as opposed to idling caused by unexpected delays that are not location-specific, such as one-off track obstructions due to wildlife.

The utilization of an automatic shutoff system was one of the EPA’s proposed post-1998 control measures that they identified to not have a significant impact on “the design or manufacture of a new locomotive or engine is the retrofitting of an auxiliary power unit,” but would have possibly been preempted under the prior regulation.[26] Under the Final Rulemaking, the EPA has deleted the language of 40 C.F.R. § 1074.12(b) (2023) that may have previously prevented California from implementing the Idling Requirement.[27] Prior to California’s regulation, the EPA idling restrictions that were established in 2008 only applied to “locomotive engines built or remanufactured after 2012,” meaning that trains with the least emissions controls and those with zombie engines were unregulated for idling.[28] The implementation of California’s Idling Requirements will thus drastically cut back on the amount of time that older and more harmful locomotive engines are kept running.

The track ahead for rail emission regulation will be long and winding. While the EPA’s rulemaking suggests that the Agency is open and willing to work with states on non-new locomotives and locomotive engines, the case-by-case approval process will undoubtedly take time. Moreover, California’s dedication to cleaner transportation and protecting its citizens against the varying harms of emissions should lead to a significant degree of emission reduction that other states may be able to piggyback off of.


[1] Evergreen Action Policy Team, President Biden’s Narrow Window to Accelerate Heavy-Duty Decarbonization, Evergreen Action (Aug. 21, 2023),

[2] Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2021, EPA (Apr. 13, 2023),

[3] Regulations for Emissions from Locomotives, EPA (last updated Apr. 12, 2023),

[4] See generally Greenhouse Gas Emissions Standards for Heavy-Duty Vehicles—Phase 3, 88 Fed. Reg. 25926 (Apr. 27, 2003) (codified at 40 C.F.R. pts. 1036, 1037, 1054, 1065, 1074) [hereinafter EPA’s Proposed Heavy-Duty Emissions Standards].

[5] See generally id.

[6] Locomotives and Locomotive Engines; Preemption of State and Local Regulations, 88 Fed. Reg. 77004, 77004 (Nov. 8, 2023) (codified at 40 C.F.R. pt. 1074) [hereinafter EPA’s Final Locomotive Preemption].

[7] Id. at 77005.

[8] California’s carve-out in the CAA (Section 209(e)(2)) is unique in that Congress requires the EPA to allow California to seek a waiver from federal preemption and apply its own regulations on vehicle emissions due to the State’s public health crisis from extreme smog in the 1960s and its pre-existing regulatory apparatus to deal with air pollution prior to the CAA. Vehicle Emissions California Waivers and Authorizations, EPA (last updated Oct. 20, 2023),

[9] EPA’s Final Locomotive Preemption, supra note 6, at 77007.

[10] Id.

[11] Id.

[12] Id.

[13] Id.

[14] Id. at 77006.

[15] Id.

[16] Id. at 77008.

[17] Id.

[18] Sophie Austin, California passes 1st-in-nation emission rules for trains, Associated Press News (Apr. 20, 2023),

[19] Cal. Code Regs. tit. 13, § 2478.5(a) (2023). “Original Engine Build Date” is defined as “the date of final assembly of the Locomotive Engine, prior to any Remanufacture of the Locomotive Engine.” The date is also based on the oldest Locomotive Engine that is propelling the Locomotive. Cal. Code Regs. tit. 13, § 2478.3 (2023).

[20] Cal. Code Regs. tit. 13, § 2478.5(a)(1) (2023).

[21] “Locomotive Operator” is a defined term in the California Rule, that “means the Person that is responsible for Operating in California one or more Locomotives.” The term also applies to the owner, lessee, or rentee “means the Person that is responsible for Operating in California one or more Locomotives.” Cal. Code Regs. tit. 13, § 2478.3 (2023).

[22] Austin, supra note 18.

[23] The exceptions for the Idling Requirements relate to necessary maintenance, such as preventing engine coolants from freezing and to maintain air pressure for brakes, along with staying in compliance with other federal or state regulations. Cal. Code Regs. tit. 13, § 2478.9(a) (2023).

[24] Focus on Locomotive Idling, State of Washington – Department of Ecology (Sept. 2008),

[25] Id.

[26] EPA’s Proposed Heavy-Duty Emissions Standards, supra note 4, at 26095.

[27] EPA’s Final Locomotive Preemption, supra note 6, at 77006.

[28] Ben Goldman, Cong. Rsch. Serv., IF10978, Locomotive Idling, Air Quality, and Blocked Crossings (2002).