U.S. rail workers are threatening to strike just before the holidays, potentially hobbling a national economy that has yet to fully recover from pandemic-inspired supply-chain troubles. Rumblings of a possible work stoppage are growing louder after a union of track maintenance workers rejected a tentative labor agreement earlier this week.

It’s the second time since July that there’s an impending rail strike, as workers continue to express concern over their existing labor contract.

A strike could happen as early as Nov. 19, just before the nation’s busiest shopping season, and analysts say it could hamper passenger service and slow the flow of goods to stores and warehouses. A union strike could cost the U.S. economy up to $2 billion per day, according to an estimate from the Association of American Railroads. Roughly 40% of the nation’s long-distance trade is moved by rail—more than any form of transportation—and a union strike could stop more than 7,000 trains from operating.

A strike would also disrupt a sizable amount of passenger traffic since Amtrak and many other commuter railroads operate on tracks owned by the freight railroads. In September, Amtrak canceled all of its long-distance trains, ahead of an earlier strike deadline out of precaution.

Here’s what to know about the potential rail strike.

What rail workers want

Members of the Brotherhood of Maintenance of Way Employees Division (BMWED), the nation’s third-largest rail union, rejected a proposed five-year contract on Monday that would have raised wages and provided an additional paid day off for rail workers. Some workers felt it didn’t go far enough, and the union vetoed the contract by a vote of 56% to 43%. Union leaders said they pushed for 15 paid sick days, but the proposed deal included only one. Rail workers are also seeking the right to take unpaid sick days, and the deal did not address this issue, among other concerns, Union President Tony Cardwell said in a public statement on Monday.

“Railroaders are discouraged and upset with working conditions and compensation and hold their employer in low regard. Railroaders do not feel valued,” Cardwell said. “They resent the fact that management holds no regard for their quality of life, illustrated by their stubborn reluctance to provide a higher quantity of paid time off, especially for sickness.”

The demands reflect a changing industry that’s been strained by pandemic-era pressures. Rail industry giants, including BNSF, Union Pacific, Norfolk Southern and CSX, have been aggressively cutting costs and shifting operations to rely on fewer trains and fewer employees. Over the past six years, major railroads have cut nearly one-third of their workforce—roughly 45,000 jobs—forcing the remaining workers to often be on-call 24/7, according to the rail unions involved in contract discussions.

Read More: America’s Railroads Are in Trouble–With or Without a Strike

The rejection by the union members comes less than a month after the Biden Administration helped broker a tentative agreement—during a marathon 20-hour bargaining session—to avoid a worker strike set to take place in mid-September.

Rail operators and the unions have agreed to resume negotiations until Nov. 19, at which point union workers could go on a strike if a deal isn’t reached by then.

Railroad workers first threatened to strike in July after more than three years of failed contract negotiations. The unions have generally pushed for a pay increase, better working conditions, relaxed attendance rules, and additional paid time off without fear of punishment.

“A hundred-plus years ago there were strikes over similar issues,” says Jasmine Kerrissey, an associate professor and director of the Labor Center at UMass Amherst. “Their slogan was ‘8 hours for work, 8 hours for rest, and 8 hours for what we will.’ And, while quality of life and the ability to take time off has always been a key issue for the labor movement, COVID has certainly shined a light on the importance of sick days and health.”

What the Biden-brokered deal included

Rail operators reached the tentative agreement with unions on Sept. 15, prompting President Biden to celebrate it as a victory for both workers and railway companies. But it still needed to be ratified and voted on by rank-and-file union members.

The agreement included an immediate 14% pay raise for employees and a 24% pay increase by 2024, the biggest wage increases for railroad workers in more than four decades.

The agreement also guaranteed voluntary assigned days off and a single additional paid day off. Workers would be able to take time off for routine doctor’s appointments without being penalized, and they would not lose attendance points for hospitalizations and surgical procedures, officials said.

The plan also included $1,000 annual bonuses over five years and ensured there would be no increases to health care co-pays and deductibles, giving each union member roughly $11,000 more once union members ratified the deal and raising the average railroad worker’s pay to $110,000 a year by 2024.

How a strike ahead of the holidays can be averted

Although a strike is possible, BMWED said it agreed to delay any such activity until five days after Congress reconvenes in mid-November to allow time for additional negotiations—meaning both sides have until Nov. 19 to reach a new deal. Railroads are expected to continue operating normally in the meantime.

Labor law for railroad employees does allow Congress to take action to keep workers on the job in case of a strike or a lockout of workers by management, though it’s unclear how Congress may act after a crucial midterm election.

But in order to prevent a strike, all 12 unions representing a total of 115,000 workers must vote to ratify the contract.

Kerrissey, the UMass Amherst professor, noted that last year, striking workers at John Deere voted to reject a contract proposal negotiated by union leaders, and they continued to strike until the company offered a better deal.

“There would be a lot of incentive to quickly end a potential strike. Strikes are a last resort,” she says. “Workers don’t typically want to strike, but sometimes they exercise their right to strike because they see it as the only way to get employers to improve conditions.”