If the tests are successful, CN hopes to switch its locomotives from diesel fuel to a "cleaner" alternative.
CN has altered two of its existing diesel-fuelled locomotives to operate predominantly on natural gas. The trial run is taking place on a 480 km route between Edmonton and Fort McMurray's oilsands region.
The conversion kits have been provided by U.S.-based company Energy Conversion Inc.
It's believed that the dual fuel engine will cut carbon dioxide emissions by 30 percent during the locomotive's life cycle.
Natural gas is inexpensive and widely available in North America, which could translate into reduced operation costs for the carrier.
With files from the Canadian Press