Emission Factors for Locomotives
 

Technical Highlights
 

The Environmental Protection Agency (EPA) has established 

emission standards for oxides of nitrogen (NOx), hydrocarbons 

(HC), carbon monoxide (CO), particulate matter (PM) and smoke for 

newly manufactured and remanufactured locomotives. These standards, 

which are codified at 40 CFR part 1033, include several sets of emission standards with applicability dependent on the date a locomotive is 

first manufactured. The first set of standards (Tier 0) applies to most 

locomotives originally manufactured before 2001. The most stringent 

set of standards (Tier 4) applies to locomotives originally manufactured 

in 2015 and later. This fact sheet describes EPA’s estimates of the typical in-use emission rates for locomotives subject to these standards, as 

well as the previous standards. 

It is important to emphasize that this fact sheet relies on many simplifying assumptions. Thus emission rates calculated as described in this 

fact sheet should be considered as approximations. 

Estimated Locomotive Emission Rates by Tier 

EPA has estimated average emission rates, given in grams per brake horsepower-hour 

(g/bhp-hr), for uncontrolled locomotives and those required to meet the various 

emission standards. Emissions were estimated for two different types of operation: a 

low power cycle representing operation in a switch yard, and a higher power cycle 

representative of general line-haul operation. These estimates are shown in Tables 

1 and 2. Note that plus signs in the table indicate that a given tier of standards was 

revised in a 2008 rulemaking (73 FR 37096, June 30, 2008). For example, locomotives originally manufactured in years 2002-2004 were initially subject to the original 

Tier 1 standards, but will be required to meet revised Tier 1 standards (also known as 

Tier 1+ standards) when remanufactured. See the regulatory text for a more precise 

explanation of which standards apply to which locomotives. 

Office of Transportation and Air Quality 

EPA-420-F-09-025 

April 2009 
It is important to note that there can be significant variability in in-use emission rates, especially 

for uncontrolled locomotives. Also, a single locomotive’s emission rate can vary throughout its 

life as the engine ages and as ambient conditions change. Thus the values presented here are intended to reflect the average emission rates. It is also worth noting that these emission estimates 

were developed in the context of adopting new emission standards. This is especially important 

for the CO emission factors. Because EPA’s CO emission standards were intended to cap CO 

emissions at pre-control levels (which were relatively low), we have not projected any reductions in CO emission factors. However, recent testing indicates that emission controls designed 

to reduce PM and HC emissions are also reducing CO emissions. Thus the CO emission rates 

presented here may be too high and should be used with some caution. A similar effect may also 

apply for HC emissions from Tier 0 and Tier 1 locomotives (but not the Tier 0+ and Tier 1+ 

locomotives). 

Table 1 - Line-Haul Emission Factors (g/bhp-hr) 

PM10 HC NO x CO 

UNCONtrOlled 0.32 0.48 13.00 1.28 

tier 0 0.32 0.48 8.60 1.28 

tier 0+ 0.20 0.30 7.20 1.28 

tier 1 0.32 0.47 6.70 1.28 

tier 1+ 0.20 0.29 6.70 1.28 

tier2 0.18 0.26 4.95 1.28 

tier 2+ & tier 3 0.08 0.13 4.95 1.28 

tier 4 0.015 0.04 1.00 1.28 

+ iNdiCates tHat tHese are tHe revised staNdards iN 40 CFr Part 1033 

Technical Highlights 

PM10 HC NO x CO 

UNCONtrOlled 0.44 1.01 17.40 1.83 

tier 0 0.44 1.01 12.60 1.83 

tier 0+ 0.23 0.57 10.60 1.83 

tier 1 0.43 1.01 9.90 1.83 

tier 1+ 0.23 0.57 9.90 1.83 

tier2 0.19 0.51 7.30 1.83 

tier 2+ 0.11 0.26 7.30 1.83 

tier 3 0.08 0.26 4.50 1.83 

tier 4 0.015 0.08 1.00 1.83 

+ iNdiCates tHat tHese are tHe revised staNdards iN 40 CFr Part 1033 

Table 2 - Switch Emission Factors (g/bhp-hr)
 

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Technical Highlights
 

Conversion to Gram per Gallon Emission Factors 

It is often useful to express emission rates as grams of pollutant emitted per gallon of fuel consumed (g/gal). This can be done by multiplying the emission rates in Table 1 or 2 by a conversion factor relating the fuel consumption (gal/hr) and the usable power (bhp) of the engine. 

EPA has estimated different conversion factors for different types of locomotive service as shown 

in Table 3. The two primary reasons for the differences are variations in locomotive age and 

duty cycle. Fuel efficiency tends to be worse for older locomotive designs and for locomotives 

used in low power applications such as switching. Note that the g/gal emission factors presented 

at the end of this fact sheet can be converted back to g/bhp-hr by dividing them by the conversion factors shown here. 

Table 3 

Conversion Factors (bhp-hr/gal) 

locomotive application Conversion Factor (bhp-hr/gal) 

large line-Haul and Passenger 20.8 

small line-Haul 18.2 

switching 15.2 

Conversion to Gram per Ton-Mile Emission Factors 

In some cases, it can be helpful to express emission factors as grams emitted per ton-mile of 

freight hauled. However, this can also be very problematic because the amount of engine work 

required for each ton-mile varies significantly with a variety of factors. For example, it takes 

more work to haul freight through mountainous terrain than across flat areas. Since EPA does 

not have detailed information about these variations, we cannot provide accurate g/ton-mile 

emission rates. However, very approximate national average values can be calculated based on 

data collected by the Association of American Railroads for revenue ton-miles and fuel consumption, which show that about one gallon of fuel is consumed by the railroads to haul 400 

tons-miles of freight. Thus dividing g/gal emission rates by 400 ton-miles/gal gives approximate 

g/ton-mile emission rates. 

Emission Inventory Estimation 

Total emissions can be calculated by multiplying the emission factors (in g/gal) by the fuel consumption rates (in million-gal/yr) to give annual emission rates (in metric tons per year). Multiplying this metric estimate by 1.102 gives standard U.S. tons (or short tons) per year. 

EPA has estimated that locomotives consume approximately 4 billion gallons of diesel fuel each 

year. This includes national/regional freight service, switching, local freight service, and passenger service. The relative amounts of fuel used in the United States for these four different types 

of operation are shown in Table 4. The great majority of fuel consumed by locomotives each 

year is used in line-haul freight service by the largest railroads. Smaller amounts are also used in 


 
Technical Highlights
 

switching and passenger service, and by very small railroads. For the purpose of this fact sheet, 

we are aggregating the largest railroads with smaller railroads that are fully subject to EPA’s emission requirements. This includes regional railroads as well as other railroads such as those that 

are owned by large businesses. The local freight category includes only those railroads that meet 

our regulatory definition of “small railroad” (40 CFR 1033.901) to qualify for small business allowances under our regulations. These railroads are included in this fact sheet as local whether 

or not they are truly local in nature. The passenger category includes local commuter railroads 

and AMTRAK. 

Table 4 - Locomotive Fuel Consumption by Service Category 

National and regional Freight line-haul 88% 

National Freight switching 7% 

local Freight <2% 

Passenger 3% 

Other Pollutants 

The preceding emission factors include those pollutants for which EPA has set emission standards. However, other pollutants may also be of interest. 

The broad category of volatile organic compounds (VOC) is a slightly different way of aggregating the organic pollutants controlled by our HC emission standards. In our rulemaking analysis 

(http://www.epa.gov/otaq/regs/nonroad/420r08001a.pdf), we estimated that VOC emissions 

can be assumed to be equal to 1.053 times the HC emissions. Similarly, PM emissions can be 

expressed as PM (which includes all particles up to 10 microns in diameter) or PM (which 10 2.5

includes only those particles up to 2.5 microns in diameter). PM2.5 emissions can be estimated 

as 0.97 times the PM10 emissions, meaning that nearly all of the PM is less than 2.5 microns in 

diameter. 

Gram per gallon emissions of sulfur dioxide (SO

2

) and carbon dioxide (CO

2

) are largely independent of engine parameters and are primarily dependent on fuel properties. Locomotive-specific emission rates are not presented here. Instead, SO2

 and CO

2

 emission rates should be calculated based on the properties of the specific fuel being used by the locomotives. These emission 

rates can also be assumed to be the same as for other diesel engines operating on similar fuel. 

Note that special caution should be used when estimating SO

2

 emission rates since the sulfur 

content of diesel fuel varies much more than the carbon content. Also, while the vast majority 

of sulfur in the fuel is typically converted to SO

2

, up to 5 percent of the sulfur is oxidized further to sulfate (and forms particulate matter), so that the fraction of fuel sulfur emitted as SO2 

may be as low as 95 percent. Examples of these calculations are shown below based on inputs 

described in the NONROAD technical document NR-009c (http://www.epa.gov/otaq/models/ 

nonrdmdl/nonrdmdl2004/420p04009.pdf). 

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Technical Highlights
 

sO2 (g/gal) = (fuel density) × (conversion factor) × (64 g sO2/32 g s) × (s content of fuel) 

Consider the example where the density of diesel fuel is 3200 g/gal, the fraction of fuel sulfur 

converted to sO2 is 97.8 percent, and the sulfur content of the fuel is 300 ppm. 

sO2 (g/gal) = (3200) × (0.978) × (2.00) × (300 x 10-6) = 1.88 g/gal 

CO2

 (g/gal) = (fuel density) × (44 g CO2/12 g C) × (C content of fuel) 

Consider the example where the density of diesel fuel is 3200 g/gal and the carbon content of 

the fuel is 87 percent by mass. 

CO2 (g/gal) = (3200) × (3.67) × (0.87) = 10,217 g/gal 

Other trace pollutants such as N2O, methane, and many air toxics are more dependent on 

engine parameters. At this time, however, EPA does not have detailed emission rates for these 

pollutants from locomotives. Where estimates are needed for N2O or methane, you may assume 

that emissions of these pollutants from locomotives are similar to those of other diesel engines 

with similar technology. For N2

O, you may assume the emissions are proportional to total NOx. 

For methane, you may assume the emissions are proportional to total hydrocarbons. Note however, that the presence of catalyzed components in the exhaust can significantly affect these 

ratios. So it is best to compare emissions from uncatalyzed locomotives to emissions from other 

uncatalyzed diesel engines. While this same approach could be used for air toxics (assuming that 

air toxic emissions are proportional to total hydrocarbons), EPA has estimated air toxic emissions from locomotives. These estimates are described in the National Emission Inventory documentation (see ftp://ftp.epa.gov/EmisInventory/2002finalnei/documentation/mobile/2002nei_ 

mobile_nonroad_methods.pdf - appendix C). 

Projected Future Emission Factors 

Tables 5-7 give the expected fleet average NOx, PM10, and HC emission factors by calendar year 

for the four categories of locomotives (the same four categories as are shown in Table 4). The 

steady decline in these emission factors reflects the penetration of the various tiers of locomotives into the fleet over time. More detail regarding the assumptions on which these projections 

were based can be found in the Regulatory Impact Analysis for the 2008 rulemaking (http:// 

www.epa.gov/otaq/regs/nonroad/420r08001a.pdf) 

5 
For More Information 

You can access the rule and related documents on EPA’s Office of Transportation and Air Quality (OTAQ) Web site at: www.epa.gov/otaq/locomotives.htm. 

For more information on this rule, please contact the Assessment and Standards Division information line at: 

U.S. Environmental Protection Agency
 

Office of Transportation and Air Quality
 

2000 Traverwood Drive
 

Ann Arbor, MI 48105
 

Voicemail: (734) 214-4636
 

E-mail: asdinfo@epa.gov
 

Technical Highlights
 


 
Table 5 – NOx Emission Factors (g/gal) 

Technical Highlights
 

Calendar large line- large small Overall 

Year 

2006 

haul 

180 

switch 

250 

railroads 

242 

Passenger/Commuter 

244 

average 

188 

2007 175 249 242 229 183 

2008 169 243 242 214 177 

2009 165 241 242 200 172 

2010 157 236 242 183 165 

2011 149 235 242 167 157 

2012 144 227 242 157 152 

2013 139 225 242 147 147 

2014 135 217 242 138 143 

2015 129 215 240 131 137 

2016 121 208 239 119 129 

2017 114 206 237 112 122 

2018 108 202 236 105 117 

2019 103 200 233 98 112 

2020 99 187 231 93 107 

2021 94 185 228 88 102 

2022 89 177 225 83 97 

2023 84 172 223 78 92 

2024 79 162 220 73 87 

2025 74 150 217 68 81 

2026 69 144 215 64 77 

2027 65 138 212 60 72 

2028 61 132 209 56 68 

2029 57 126 206 52 64 

2030 53 119 203 49 60 

2031 49 112 200 46 56 

2032 46 105 197 42 52 

2033 43 98 193 39 49 

2034 40 91 190 36 46 

2035 37 84 187 33 43 

2036 35 77 184 30 40 

2037 33 71 180 28 38 

2038 31 67 177 26 36 

2039 29 63 174 24 34 

2040 28 60 171 23 32 


 
Table 6 – PM10 Emission Factors (g/gal) 

Technical Highlights
 

Calendar large line- large small Overall 

Year 

2006 

haul 

6.4 

switch 

6.5 

railroads 

6.5 

Passenger/Commuter 

6.5 

average 

6.4 

2007 6.3 6.5 6.5 6.4 6.3 

2008 5.1 5.5 5.7 5.1 5.1 

2009 4.9 5.5 5.7 5.0 4.9 

2010 4.7 5.4 5.7 4.8 4.7 

2011 4.4 5.3 5.7 4.5 4.5 

2012 4.1 5.1 5.7 4.2 4.2 

2013 3.8 5.0 5.6 3.9 3.9 

2014 3.6 4.8 5.6 3.6 3.7 

2015 3.4 4.8 5.5 3.4 3.5 

2016 3.1 4.6 5.5 3.1 3.3 

2017 2.9 4.5 5.4 2.8 3.0 

2018 2.7 4.4 5.4 2.6 2.8 

2019 2.5 4.4 5.4 2.3 2.6 

2020 2.3 4.1 5.3 2.1 2.5 

2021 2.2 4.0 5.3 2.0 2.4 

2022 2.0 3.9 5.3 1.8 2.2 

2023 1.9 3.7 5.2 1.7 2.1 

2024 1.7 3.5 5.2 1.5 1.9 

2025 1.6 3.2 5.1 1.4 1.8 

2026 1.5 3.1 5.1 1.2 1.6 

2027 1.4 3.0 5.1 1.1 1.5 

2028 1.3 2.8 5.0 1.0 1.4 

2029 1.1 2.7 5.0 0.9 1.3 

2030 1.0 2.5 4.9 0.8 1.2 

2031 1.0 2.4 4.8 0.7 1.1 

2032 0.9 2.2 4.8 0.7 1.0 

2033 0.8 2.1 4.7 0.6 0.9 

2034 0.7 1.9 4.6 0.6 0.9 

2035 0.7 1.7 4.6 0.5 0.8 

2036 0.6 1.6 4.5 0.5 0.7 

2037 0.6 1.5 4.4 0.4 0.7 

2038 0.5 1.4 4.4 0.4 0.6 

2039 0.5 1.3 4.3 0.4 0.6 

2040 0.4 1.2 4.2 0.3 0.5 


 
Table 7 - HC Emission Factors (g/gal) 

Technical Highlights
 

Calendar large line- large small Overall 

Year 

2006 

haul 

9.5 

switch 

15.0 

railroads 

11.7 

Passenger/Commuter 

9.7 

average 

10.0 

2007 9.3 15.0 11.7 9.5 9.8 

2008 9.0 14.5 11.7 9.3 9.5 

2009 8.7 14.5 11.7 9.1 9.1 

2010 8.3 14.1 11.7 8.6 8.8 

2011 7.7 14.0 11.7 8.1 8.2 

2012 7.1 13.3 11.7 7.5 7.6 

2013 6.5 13.3 11.7 6.9 7.1 

2014 6.1 12.7 11.7 6.3 6.7 

2015 5.7 12.6 11.7 5.8 6.3 

2016 5.1 12.0 11.7 5.2 5.7 

2017 4.6 11.8 11.7 4.6 5.2 

2018 4.2 11.5 11.7 4.1 4.8 

2019 3.9 11.4 11.7 3.5 4.5 

2020 3.6 10.5 11.7 3.1 4.2 

2021 3.4 10.4 11.7 2.9 4.0 

2022 3.2 9.8 11.7 2.7 3.8 

2023 3.0 9.5 11.7 2.4 3.6 

2024 2.8 8.9 11.7 2.2 3.4 

2025 2.6 8.0 11.7 2.0 3.1 

2026 2.5 7.6 11.7 1.8 2.9 

2027 2.3 7.3 11.7 1.6 2.8 

2028 2.1 6.9 11.7 1.5 2.6 

2029 2.0 6.5 11.7 1.3 2.4 

2030 1.9 6.2 11.7 1.2 2.3 

2031 1.7 5.8 11.7 1.1 2.2 

2032 1.6 5.5 11.7 1.0 2.0 

2033 1.5 5.1 11.7 0.9 1.9 

2034 1.4 4.7 11.7 0.8 1.8 

2035 1.3 4.4 11.7 0.7 1.7 

2036 1.2 4.0 11.7 0.7 1.6 

2037 1.2 3.7 11.7 0.6 1.5 

2038 1.1 3.6 11.7 0.6 1.4 

2039 1.1 3.4 11.7 0.5 1.4 

2040 1.0 3.2 11.7 0.5 1.3 

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