By MATTHEW BROWNBILLINGS, Mont. (AP) â¿¿ Opponents of a railroad that would open Montana's coal fields to new mining have asked federal officials to halt their review of the proposal, alleging it would be much larger than disclosed with impacts stretching to the West Coast. Backers of the Tongue River Railroad say it would haul up to 20 million tons annually from a planned Arch Coal Inc. strip mine near the Northern Cheyenne Indian Reservation. But opponents said Thursday they want to turn the spotlight on the potential for the $416 million railroad to spur more mining of coal for export to Asia, along a route through Montana, Idaho and Washington state. They allege the railroad's owners intentionally played down the scope of the project, by saying it would haul an average of 7.4 full and empty coal trains a day. That figure counts only Arch's immediate mine plan. A second spur proposed by the railroad would give it access to additional coal fields west of Arch's mine. At least one mine, the Montco Mine, was previously proposed in that area. The Northern Cheyenne have also had discussions with companies about mining. Attorneys for the railroad acknowledged the potential for additional mines in their Dec. 17 application for federal approval. They did not provide any potential volumes. A spokeswoman for BNSF Railway said she could not immediately respond Thursday. Arch referred questions to BNSF. If such mining took place, traffic on the Tongue River Railroad would quickly top the government's threshold of eight trains daily needed to trigger a more intensive environmental review. That would include an air pollution analysis. 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.