The U.S. Supreme Court on Tuesday will hear arguments in a yearslong legal battle over a contentious proposed railroad that could send tens of millions more gallons of crude oil along the Colorado River, including near the critical water source’s headwaters in Colorado.

Opponents of the 88-mile Uinta Basin Railway — led by Colorado’s Eagle County — successfully halted the Utah project when a federal appeals judge in 2023 agreed with their arguments that the potential environmental impacts of the rail line had not been sufficiently analyzed.

But the coalition of private companies and seven Utah counties supporting the project petitioned the Supreme Court to review the appeals judge’s decision. In June, the nation’s highest court accepted their petition and said it would consider how far federal agencies must go in analyzing potential environmental impacts.

If constructed, the rail line would connect Utah’s oil fields to the national rail network. It would greatly increase the amount of crude oil transported across Colorado and on to the refineries on the Gulf Coast.

The project would allow oil producers to send up to 350,000 barrels of crude oil a day on nine more trains — each stretching as long as two miles — on the tracks along Colorado’s Interstate 70, which follows the Colorado River for hundreds of miles.

A dozen local governments in Colorado, across political divides, and the state’s attorney general filed briefs in the case urging the Supreme Court not to change the decision issued by the U.S. Court of Appeals for the D.C. Circuit. They also argue in favor of maintaining the federal environmental law in question, the National Environmental Policy Act.

“The project raises the risk of leaks, spills, or rail car accidents immediately adjacent to the headwaters of the Colorado River, the most critical water source for the state’s residential communities, and agricultural and outdoor recreation sectors,” Attorney General Phil Weiser‘s brief states. “The project’s risks to Colorado’s residents and natural resources have generated deep concern and strong opposition from across the state.”

The justices will hear from both sides Tuesday morning, but a decision is likely weeks or months away. Justice Neil Gorsuch last week recused himself from the case after ethics watchdogs noted his ties to Colorado billionaire Philip Anschutz, whose companies could profit if the railway is built, according to reporting from The New York Times.

Construction on the project could not begin even if the Supreme Court sides with the railway project because the court is analyzing only one of the reasons the lower court halted the railway. If the Supreme Court agrees with railway proponents’ arguments, the lower court will have to reassess its analysis and issue a new opinion.

But conservationists fear the court could use the case to weaken one of the nation’s foundational environmental protection laws.

The decision could lead to “a radical restriction of the way the government evaluates the environmental impact of decisions,” said Sam Sankar, the senior vice president of programs for the environmental legal group Earthjustice.

At the heart of the litigation is whether the U.S. Surface Transportation Board — a federal agency that regulates railways — violated the National Environmental Policy Act by failing to analyze potential environmental impacts of the project outside of the immediate area of the proposed line. The federal appeals judge last year found that the board had violated the law and should have scrutinized potential threats to the Colorado River as well as increased wildfire risk caused by more train traffic.

Lawyers for the Seven County Infrastructure Coalition, which is spearheading the rail project, argued that such a thorough analysis was not needed. The Surface Transportation Board should consider the immediate environmental impacts of a project, such as whether construction will displace bighorn sheep habitat or alter a mountain stream, the attorneys argued in their brief to the U.S. Supreme Court.

But the board should not be required to analyze “imponderables such as whether the new rail might contribute to an accident hundreds or thousands of miles downline,” they wrote.

“If a new rail line in Utah will displace habitat for bighorn sheep or alter the topography in ways that threaten a pristine mountain stream, the Surface Transportation Board must consider those issues,” the brief states. But the chance of a faraway crash or that “the new rail might … somehow affect ‘environmental justice (in) communities (on) the Gulf Coast’ are not issues the Surface Transportation Board must run to ground.”

The Colorado communities opposed to the new railway include Glenwood Springs, Grand County, Grand Junction, Avon, Basalt, Routt County and the Northwest Colorado Council of Governments, which represents 31 counties and municipalities in the northwest region of the state.

They reject arguments that impacts on their communities shouldn’t be considered. Glenwood Springs city councilman Jonathan Godes said any spill of crude oil into the Colorado River would be catastrophic for his town and every community downstream.

“Our entire recreation economy is built around the river,” he said. “It would destroy our economy and our drinking water.”

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