The country’s two main railways are starting to block shipments ahead of a possible strike or lockout next week.

Canadian National Railway Co. says in an internal memo obtained by The Canadian Press that the company has begun to embargo some hazardous goods from the U.S. in anticipation of a work stoppage.

CN says it plans to bar more commodities this week if no agreement with workers is reached.

On Friday, Canadian Pacific Kansas City Ltd. said it will temporarily ban traffic of dangerous materials to ensure none wind up stranded on the tracks in the event of job action.

The two railways warned last week they will lock out some 9,300 engineers, conductors and yard workers on Aug. 22 unless they find common ground on new contracts after negotiations stalled over scheduling and wages.

Shippers and producers say a work stoppage would snarl the country’s supply chain, halting freight traffic and disrupting industries.