The Amalgamated Transit Union had planned to set a strike deadline for Monday but that has been delayed because the company has gone to the Ontario Labour Board.
ATU Local 1587 president Ray Doyle says Metrolinx -- the provincial agency that runs GO Transit -- wants the board to overturn a letter of agreement the two sides signed in March.
Doyle wouldn't divulge the details of the letter of agreement but says the union will apply to the board no later than Thursday seeking an expedited hearing.
When asked why the company went to the labour board, Metrolinx said it doesn't discuss its negotiations publicly.
Both sides say they are committed to trying to reach a deal avoid a strike that would idle GO buses but not GO trains.
Doyle says key issues are the company's offer of a wage freeze over two years and working conditions.