Rail Strike Threat Dredges Up New Fears of US Supply-Chain Havoc

In a short time, a critical cog of the American economy could come to an abrupt halt.

Unions and freight railroads that transport about 40% of all long-haul cargo in the US have until the end of Thursday in Washington to agree to a labor deal. If they fail, more than 100,000 railroad workers could walk off the job or be locked out, paralyzing a linchpin of the country’s supply chain.

The risk of a work stoppage has producers of everything from airplanes and autos to diapers girding for yet another shock that will only add to the supply-chain ...