Thousands of railway employees could be off the job once again on Monday morning.

The union representing CN Rail workers issued a 72-hour strike notice on Friday.

“Please find this letter as official notice to the company of our intention to withdraw the services of our combined membership of approximately 6,500 members,” said the notice from Teamsters Canada Rail Conference.

“We do not believe that any of the matters we have been discussing over the last several days are insurmountable and we remain available for discussion in order to resolve this matter without a further work stoppage.”

The strike notice came hours after the union said workers would begin returning to work after a stoppage the day prior.

Around 9,300 workers at CN and Canadian Pacific Kansas City were locked out on Thursday after negotiations failed to produce a new agreement.

Later that same day, federal Labour Minister Steven MacKinnon announced the matter would go to binding arbitration to help the sides hammer out a deal.

CN announced in a statement that it ended its lockout as of 6 p.m. Eastern Time on Thursday and immediately initiated its recovery plan while awaiting a formal order from the Canada Industrial Relations Board (CIRB).

Meanwhile, the work stoppage at CPKC remains ongoing pending an order from the board.

“Despite the Labour Minister’s referral, there is no clear indication that the CIRB will actually order an end to the labour dispute at CPKC,” the union said in a statement on Thursday.

The union and railroad officials met with the board on Thursday and were scheduled to meet again Friday.