MONTREAL/OTTAWA -- Canadian National Railway Co (CNR.TO) and a union representing 3,000 conductors on Monday reached an agreement in principle ahead of a looming strike deadline at Canada's largest railroad, said a top union official.

The union had set a strike deadline of Tuesday at 4 a.m. ET after the railroad announced new work rules during negotiations to replace an expired contract.

"They are working on the final language," Teamsters Canada Rail Conference president Doug Finnson said by telephone.

The stoppage would have been be the first strike by CN conductors, or train operators, in a decade. Two sources close to the talks had earlier reported good progress.

Rail shippers had earlier urged Ottawa to intervene, fearing that a shut-down would immediately damage business.

Freight Management Association of Canada wrote to Labour Minister Patty Hajdu on Sunday asking her to impose binding arbitration to resolve the dispute.

Hajdu, speaking to reporters on Monday, did not respond to questions about whether she was considering legislation to keep trains running during a strike. A federally appointed mediator is assisting in the talks.

"We have every confidence that we're going to get a deal," she said.

Canada exports most of the grain, potash and other commodities that it produces, moving them vast distances to ports via CN or Canadian Pacific Railway Ltd (CP.TO).

A strike would delay arrival of commodity shipments at port, racking up higher costs, said freight association president Bob Ballantyne on Monday. Retail importers would be affected, along with auto manufacturers who rely on just-in-time parts delivery, he said.