Amtrak is preemptively canceling trips on three of its long-distance routes that operate on freight rail-owned tracks and adjusting schedules on other routes due to a possible strike by two of the country’s largest rail unions. 

The rail operator, whose workforce is not involved in the labor talks, is “closely monitoring” the ongoing negotiations, it said Monday. The suspension of service on the three routes as of Tuesday could spread to all long distance and most state-supported routes. 

“Amtrak has now begun phased adjustments to our service in preparation for a possible freight rail service interruption later this week. Such an interruption could significantly impact intercity passenger rail service,” it said in an emailed statement. 

Freight-rail operators and unions face a Friday deadline to agree to a new contract. The two sides worked through the weekend in a bid to hammer out a new contract to avoid a strike that could snarl supply chains, disrupt agricultural deliveries and cost the US economy more than $2 billion a day. 

The company said most service within its own Northeast Corridor linking Boston, New York and Washington -- including its Acela trains -- will continue to operate on a full schedule, but that nearly all of the 21,000 route miles it operates outside of that area uses railway track “owned, maintained and dispatched by freight railroads.”