While Union Pacific Chief Executive Officer Lance Fritz is not directly involved in the sixth round of talks in Montreal, he said he has been "constantly in contact" with appointed and elected U.S. officials close to the negotiations.

"If the U.S. is intransigent, that is a concern," Fritz said by phone after the No. 1 U.S. railroad by revenue reported a higher quarterly profit. "What I am saying is, I don't know that as a fact. I would imagine that what you hear from negotiations, when it's down to crunch time, might be kind of posturing, might be manipulating – who knows – it's a concern."

"I am still hopeful that all involved can get to the right conclusion," Fritz said.

U.S. negotiators are holding firm in their demands for a wide-ranging overhaul of the 1990s-era trade pact between the United States, Canada and Mexico, sources close to the talks said, raising questions about whether there is any real movement with major differences that must be settled by the end of March.

"My concerns are that the U.S. Trade Representative's Office won't get to the right spot without a collaborative, creative, diligent approach with the other trade reps to secure the future of NAFTA," Fritz said.

U.S. President Donald Trump has threatened to withdraw from NAFTA, which the U.S. and Canadian railroads rely on to haul freight such as cars, beer, and grain across the three countries.

"NAFTA is not a failed agreement," Fritz said. "It's a very effective agreement to this point and it's one that needs to be modernized."

About 35 percent of annual U.S. rail industry revenue is directly linked to international trade, according to the Association of American Railroads lobby group.

(Reporting by Eric M. Johnson in Seattle; editing by Grant McCool)

By Eric M. Johnson