Background

The railroad industry is exploring the use of 

natural gas in liquid form (LNG) and in a 

compressed gas form (CNG) as alternative 

and supplemental locomotive fuels to diesel 

for environmental and economic benefits.

The Federal Railroad Administration’s (FRA) 

Office of Railroad Safety (RRS) promotes 

innovative technologies including use of 

alternative fuels. RRS also regulates safety 

throughout the nation’s railroad industry. 

RRS executes its regulatory and inspection 

responsibilities through a diverse staff of 

railroad safety experts.

More Information

For more information on LNG and CNG, visit 

the following websites:

LNG

• Department of Energy, Natural Gas (https://

www.energy.gov/natural-gas)

• Center for Liquefied Natural Gas, Cool, Clean 

Fuel (http://www.lngfacts.org/)

• California Energy Commission, Frequently 

Asked Questions about LNG (http://

www.energy.ca.gov/lng/faq.html)

• LNG Tenders/Locomotives (https://

www.fra.dot.gov/Page/P0714)

CNG

• California Energy Commission, 

Transportation Natural Gas in California

(https://www.energy.ca.gov/almanac/

transportation_data/cng-lng.html)

• LNG Tenders/Locomotives (https://

www.fra.dot.gov/Page/P0714)

Liquid and 

Compressed 

Natural Gas as 

Locomotive Fuels
How it Works

• Liquefied natural gas (LNG) results when 

natural gas (NG) is cooled to very low 

temperatures at atmospheric pressure. 

LNG is stored in highly insulated, doublewalled tanks. LNG, as a liquid, provides 

significant reduction in shipping and 

storage volumes when compared to NG of 

the same mass in a gaseous state at 

atmospheric temperature and pressure 

conditions. 

• LNG when spilled, or vaporized in 

vaporizers, is converted into NG.

• LNG spills and leaks lead to the formation 

of a cold NG cloud. This vapor cloud is 

visible due to water vapor in the air 

condensing into the LNG cloud as a fog.

• The NG cloud formed by LNG vaporization, 

when mixed with air, is flammable; but it 

will ignite only when the gas-air mixture is 

within a specific concentration range.

• Compressed natural gas (CNG) used as a 

locomotive fuel is stored at high pressures 

(>250 atmospheres) and ambient 

temperatures in cylinders. These cylinders 

are carried either on the locomotive or on 

a tender behind the locomotive.

• Locomotives using alternative fuels (LNG or 

CNG) have engines that burn NG and 

diesel fuel (dual-fuel operation).

• Normally, a locomotive uses the alternative 

fuel as the predominant source; however, 

diesel is used at very low engine loads, and 

as an igniter of the alternative fuel in the 

engine.

• A dual-fuel engine will burn diesel-only in 

the event of NG-fuel feed failure or in the 

event of deliberate NG fuel feed shut off or 

interruption.

Safety

• Dual-fuel NG locomotives have safety 

systems designed to protect railroad 

personnel and first responders in case of 

an accident.

• FRA requires a comprehensive safety 

analysis before the initiation of alternative 

locomotive fuel projects.

• FRA is continuing to research safety issues 

related to LNG and CNG.

Benefits

• Greenhouse gas emissions from the 

combustion of NG are much lower than 

those of diesel for the same energy output. 

This results in less carbon dioxide and 

other pollutants being released into the 

atmosphere.

• NG is widely available in the United States, 

and historically, has been cheaper than 

diesel fuel. Therefore, switching to LNG 

could save money in fuel costs.

Contact Us

Dr. Phani Raj

General Engineer

Hazardous Material Division

(202) 493-6306

Phani.Raj@dot.gov