Under federal law, that would trigger a more intensive environmental review, including an air pollution analysis, if Tongue River Railroad was preparing the environmental study of the project. However, because the Surface Transportation Board is leading the study, it's uncertain if that threshold applies.

Others want the application rejected outright. Citing the potential for increased train traffic, the Rocker Six Cattle Co. and Northern Plains Resource Council this week petitioned the Surface Transportation Board to reject the railroad's application.

Their attorney Jack Tuholske, said the railroad "fudged the numbers by not disclosing the potential volumes" from the second spur.

"They are trying to fast-track this with minimal analysis," he said. "That's why they didn't disclose the full purpose of the railroad. The route now points like an arrow to the West Coast."

The prospect of increased coal exports also has sparked interest in Washington state. The director of the Washington Department of Ecology urged federal officials in a letter last week to consider a range of far-reaching impacts from the railroad. That includes train traffic tying up rail crossings, fugitive coal dust and air pollution from locomotives.

"Additional rail being built specifically for coal trains to export coal could affect Washington State resources," Director Ted Sturdevant wrote. "It is imperative that the (Surface Transportation Board) consider the direct, indirect and cumulative impacts."

The Surface Transportation Board this week said it had accepted the railroad proposal for consideration. An agency spokesman said Thursday that the board will consider the opponents' petition during its review of the railroad.

First proposed in the 1980s, backers of the railroad struggled for years to line up financing until being bought out in 2011 by BNSF, Arch Coal and candy-industry billionaire Forrest Mars Jr.

Last month, the co-owners unveiled a new route for the line, through Colstrip instead of Miles City.

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