BOISE - Thousands of miles of active railroad tracks in 11 Western states cross public land on 200-foot-wide rights of way granted under an 1875 law intended to encourage westward expansion.

But the U.S. Department of the Interior's Office of the Solicitor determined it made a mistake 15 years ago by allowing too much discretion on what modern-day railroads can build or allow third parties to build on that land.

A proposed water pipeline in California having nothing to do with railroad operations prompted the agency to fix the 1989 error that could also be used to put in oil or natural gas pipelines.

In mid-August, the U.S. Bureau of Land Management issued a memorandum to field offices about the change that requires rights of way work to serve railroad operations only. The agency anticipates that up to 3,500 evaluations will be needed to determine if work done the last decade and a half meets the updated criteria.

"Given the linear nature of these rights of way, we would anticipate applications for comparable uses for such things as pipelines and fiber optics," said Bureau of Land Management spokeswoman Celia Boddington, who is based in Washington, D.C.

It's not clear what might already be in place in the rights of way, said Jennifer Whyte, a realty specialist for the bureau who is also based in Washington, D.C. "We don't know exactly what's in those because we haven't been involved in any of that because of the 1989 solicitor's opinion," she said.

In a new opinion in 2011, the Solicitor's Office said the 1989 opinion's "interpretation of the 1875 Act is inconsistent with the Act itself." It's also not consistent with U.S. Supreme Court rulings, the 2011 opinion said.

The General Railroad Right-of-Way Act of 1875 replaced earlier rights of way acts that started in the 1850s and followed Congressional policy by providing grants of land from the public domain to spur railroads to build rail lines in the West.

But giving away land to railroads fell out of favor, causing lawmakers to create the more limited 1875 Act that kept the land under federal ownership but still encouraged westward expansion by granting rights of way.

Federal agencies aren't sure how many miles of rights of way remain in use today from the 1875 Act.

One of the rights of way belongs to the Arizona and California Railroad Company. In 2009, federal officials relying on the 1989 opinion determined the company didn't need federal authorization or analysis to put a water pipeline within the rights of way. A third-party company wants to bury the pipeline to move groundwater from under the Mojave Desert to southern California.

Several other railroad companies said they would need to review operations to get a better understanding on whether any of their projects could be affected by the change.

"We are in the process of analyzing the new guidelines now," said Aaron Hunt, a spokesman for Union Pacific Railroad.

Burlington Northern Santa Fe spokesman Gus Melonas said the part of the company that dealt with land issues would have to examine the change before commenting.

Boddington said the next step in the process is for the Bureau of Land Management to enter into the Federal Register, the repository for federal regulations, a notice of the change. "We're drafting the notice right now, which will likely be published next year after internal review," she said.

Once published, Boddington said, railroad companies will have 180 days to provide information to the Bureau of Land Management. If the work falls outside of normal railroad operations, the bureau will work with the railroads and third parties to determine what additional authorization, if any, is needed.

"It's all going to be on an individual, case-by-case evaluation and determination," Whyte said.