DENVER — Canada’s two largest railroads are beginning to close their transportation networks as a labor dispute with the Teamsters union threatens to cause lockouts or strikes that would disrupt cross-border trade with the United States.

Both Canadian Pacific Kansas City and Canadian National Railroad, which transport millions of tons of freight across the border, have stopped accepting certain shipments of hazardous materials and refrigerated products.

Both are threatening to lock out Teamsters workers in Canada starting Thursday if no agreement is reached.

On Tuesday, CPKC will stop all shipments originating in Canada, as well as all shipments originating in the United States and destined for Canada, the railway company announced on Saturday.

The Canadian Press reported that Canadian National banned the import of containers from its U.S. partner railways on Friday.

Jeff Windau, industrial analyst at Edward Jones and Co. said that while his company expects the work stoppages to last only a few days, if they last longer, there could be significant disruptions to the supply chain.

“If something were more long-term in nature, I think there would be some significant potential problems, just given the amount of goods that are handled every day,” Windau said. “By and large, rail touches pretty much the entire economy.”

The two railways move about 40,000 carloads of freight worth about $1 billion a day, Windau said. The transport of complete cars and car parts, chemicals, forest products and agricultural goods will be severely affected, he said, especially with the upcoming harvest season.

Both railroads have extensive networks in the United States, and CPKC also serves Mexico. Operations will continue even if work is halted.

CPKC said it remains committed to avoiding a work stoppage that would harm Canada’s economy and international reputation. “However, we must take responsible and prudent steps to prepare for a possible disruption to rail service next week,” spokesman Patrick Waldron said in a statement.

By closing the network, the railway company can remove dangerous goods from its network before disruptions occur, CPKC said.

Union spokesman Christopher Monette said in an email Saturday that negotiations were ongoing but the situation had shifted from a possible strike to an “almost certain lockout” by the railroad company.

CPKC said negotiations with the union, which represents nearly 10,000 workers at both railroads, are set to resume on Sunday. The company said it continues to negotiate in good faith.

Canadian National said in a statement on Friday that no significant progress had been made in negotiations and that it hoped the union would “meaningfully engage” at a meeting scheduled for Saturday.

“CN wants a solution that allows the company to refocus on what it does best as a team: transporting customer goods and the economy,” the railroad company said.

Negotiations have been ongoing since last November and the contracts would have expired at the end of 2023. As the talks progressed, they were extended.

The union said the company’s demands regarding workforce planning, rail safety and worker fatigue were the biggest points of contention.

Concerns about the quality of life of rail workers struggling with demanding hours and no paid sick leave nearly led to a strike by U.S. rail workers two years ago, but Congress intervened and prevented a strike. Major U.S. railroads have since made progress by offering paid sick leave to most rail workers and trying to improve work hours.

Windau said that the trucking industry currently has large excess capacity and could possibly cover part of the rail transport volume. However, “it will not be possible to replace all of this with trucks.”