Locomotive makers have not yet set the prices of their retrofit kits, but railroads expect they will be cheaper than a new locomotive costing roughly $2 million.
Using both diesel and natural gas also offers some advantages over using natural gas alone. The diesel can provide the spark needed to ignite natural gas without redesigning locomotive engines, and the diesel helps provide horsepower.
Railroads are planning to use liquefied natural gas, which is not as readily available as other forms of gas because it must be cooled to minus 260 degrees. That step adds to the price, but the amount varies based on how the process is done.
"It's so early in this that we're still working to understand the potential savings," said Louis Renjel, vice president of strategic infrastructure at CSX railroad.
The projected cost comparisons do not include the millions of dollars railroads would have to spend on a network of natural gas fueling stations along their tracks. That expense won't be clear until after the tests, when railroads decide whether to build their own liquefaction facilities or just store fuels.
"There are a lot of factors that aren't accounted for yet," said Michael Iden, who oversees locomotive engineering at Union Pacific.
Locomotive makers say natural-gas engines could also significantly reduce emissions compared with diesel locomotives, but the potential cost savings is the biggest reason the rail industry is eager to make the change.
From the outside, natural gas locomotives will not look much different, but they will have to pull a tank car behind the engine to carry enough liquefied natural gas, or LNG, to have a similar range to diesel units.
Both of the major locomotive manufacturers, General Electric and Caterpillar's Electro-Motive Diesel, have developed prototypes that will be tested by Union Pacific, CSX, BNSF and Canadian National railroads beginning this year.