WASHINGTON—Amtrak and the Biden administration have a $75 billion plan to transform passenger train travel across America—but aside from funding, the biggest challenge might be getting the nation’s freight railroads to get on board.

The nation’s seven major freight carriers have long resisted calls to make more room on their tracks for trains carrying people rather than coal, grain and steel. Current and former federal officials say the greatest impediment to Amtrak’s national expansion could be finding a way to work with freight carriers.

Amtrak owns most of the tracks where it currently offers its flagship service—the Northeast Corridor between Washington and Boston—but it relies in most of the rest of the country on access to the tracks owned by freight railroads.

That arrangement is a result of Amtrak’s DNA. Congress created the company in 1970 as railroads faced financial ruin. Amtrak bailed the railroads out of the least profitable portion of their business: moving people from place to place.

By law, Amtrak trains can take precedence over freight when running over those networks. In practice, according to Amtrak, its customers frequently suffer delays caused by freight-train interference. The two sides have battled for years over new standards for on-time performance.

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In interviews, freight and Amtrak executives said the two sides work cooperatively to balance passenger and freight traffic. In practice, conflicts are frequent.

Amtrak is seeking legal changes to force the freight carriers to provide better access for passenger trains, and some lawmakers are ready to intervene. The House Transportation Committee’s latest authorization bill would grant Amtrak the right to ask a federal court to adjudicate disputes with freight railroads, rather than seeking relief from the existing regulator, the Surface Transportation Board.

“Right now they’ve got it the way they want it,” said Rep. Peter DeFazio (D., Ore.), the committee chairman, in a recent interview, referring to freight railroads. “So we’re going to change the law and give Amtrak better access.”

Jessica Kahanek, a spokeswoman for the Association of American Railroads, a trade group that includes Amtrak and the freight railroads, said Amtrak’s success relies on the freight railroads, since the passenger railroad runs on their tracks across most of the country.

“Passenger and freight railroads share the same goal: dependable, on-time passenger service coupled with efficient, reliable freight service,” she said.

Amtrak’s decision to file a petition to compel access to rail lines along a Gulf Coast route took major freight companies, including CSX, by surprise.

Amtrak’s decision to file a petition to compel access to rail lines along a Gulf Coast route took major freight companies, including CSX, by surprise.

Photo: Luke Sharrett/Bloomberg News

Amtrak officials have said their corridor plan would use the model of regular intercity service that has worked on the Northeast Corridor to new pairs and groups of cities around the country, especially in regions where population and traffic congestion are growing the fastest.

“All we’re really proposing is to connect the major cities in the U.S. with their near neighbors and economic regions,” Amtrak President Stephen Gardner said in an interview in April.

Mr. Gardner acknowledged that there is skepticism, given that most of the country hasn’t had viable intercity train service for decades.

“But you look at the facts: What really is different about Charlotte to Atlanta and Philadelphia Washington? If you can establish something like a trip-time competitive service—there is population there, there are opportunities that can come from that service.”

With capital funding from the Biden administration’s proposed infrastructure plan, Amtrak management now says the expansion can be done without reducing service on its national network of long-distance trains—a prospect that had triggered bipartisan opposition to its corridor vision in the past.

New routes would link Phoenix and Tucson, Ariz., with Southern California. More trains would run from Oklahoma City to Fort Worth, Texas, and new corridors would link Dallas, Houston and San Antonio.

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Enhancing passenger service requires adjustments by the freight carries, and nowhere is the struggle to share space more clear than the Gulf Coast. An effort there to restore train service that was halted in the aftermath of Hurricane Katrina has turned unusually bitter, notwithstanding support from powerful politicians intent on helping people ride the rails.

In April, after years of negotiation with the freight companies that own the tracks, Amtrak filed a petition with federal regulators to compel access to the rails, allowing passenger trains to run between New Orleans and Mobile, Ala.

Amtrak’s decision to file took the major freight companies on the Gulf Coast route— CSX Corp. and Norfolk Southern —by surprise.

Shippers “are not opposed to passenger service” on the Gulf Coast, an attorney for CSX wrote in a recent Surface Transportation Board filing. “They simply want to ensure that any new passenger service will not damage the quality of existing freight service,” the attorney wrote.

Spokesmen for Norfolk Southern and CSX argued that Amtrak should continue working with an external consultant to study how passenger rail service would affect freight movement, rather than asking the Surface Transportation Board to intervene. An Amtrak spokeswoman said that the freight railroads haven’t provided “real cooperation” during years of study and that the Surface Transportation Board “is now the right forum to get this issue resolved for the people of the Gulf Coast to get the passenger train service they desire and deserve.”

In its own filing with the board, Amtrak noted that Gulf region travelers “have had to wait 15 years for the restoration of passenger rail service as CSX and NS continually move the goal posts, imposing ever-higher burdens they claim Amtrak must meet before they will agree to allow even a single passenger train to run between New Orleans and Mobile.”

The two sides are far apart on the cost it will take to share the rails.

The Transportation Department sided with Amtrak, noting that the federal government thought that $118 million in investment would be required to resume passenger service, while CSX estimated $2.3 billion.

The gap is partly from different expectations. Before Hurricane Katrina, the Sunset Limited traversed the Gulf Coast three days a week, with little conflict with freight traffic. Amtrak plans to resume service with two round trips a day between New Orleans and Mobile.

The freight railroads and shippers want to continue to study potential impacts, which they say would likely show billions of dollars would be needed to lengthen sidings to prevent passenger trains from snarling traffic.

“My fear is they’re going to start something before knowing the full impact,” said John Driscoll, director and chief executive of the Alabama State Port Authority, which runs the large and growing port in Mobile.

In a Surface Transportation Board filing, the Southern Rail Commission, an interstate panel representing Alabama, Louisiana and Mississippi, alleges that CSX has exaggerated the potential costs of restoring service.

The commission has secured $66 million in federal and local funds to improve infrastructure, including lengthening tracks and upgrading switches to allow trains to pass.

On the Gulf Coast, said Mr. Gardner, Amtrak is asking the Surface Transportation Board to decide an existential question.

“Either Amtrak’s right of access is real and we really do have rights to use the nation’s rail infrastructure, and freight carriers do have an obligation to support us, or it’s not,” he said. “It’s either real or it isn’t.”

Write to Ted Mann at ted.mann@wsj.com