The union says 81 per cent of GO employees voted against a previous contract offer from Metrolinx, leading to the walkout.
The Amalgamated Transit Union Local 1587 says its concern is with job security, and hiring connect workers from outside the company.
Metrolinx, the provincial transit agency that oversees GO Transit and the Union Pearson Express, says they will remain open to bargaining during the strike.
GO bus services will be unavailable during the strike. GO trains will continue to operate.
More information to come.