The federal government’s plan to get Canadian Pacific Kansas City (CPKC) and Canadian National Railway (CN Rail) workers back to work could go off the rails.

The Teamsters Canada Rail Conference (TCRC), which represents 93-hundred employees of CPKC and CN, issued 72-hour strike notice to Canadian National Railway Friday morning, while members on the CPKC side remain on the picket line. The TCRC is also challenging the constitutionality of the directive of binding arbitration issued by Federal Labour Minister Steven MacKinnon Thursday.

Workers held a rally outside of the CPKC Headquarters in Calgary this morning. In a statement, Canadian Pacific Kansas City expressed disappointment with the union’s challenge of the Minister’s directive and “the CIRB’s discretion to proceed with any order.”

CN trains just started moving Friday morning after a nation-wide stoppage Thursday.