The company said the termination of the contract follows a delay in the tie-in of the Cold Lake pipeline system to the North American Terminal Operations facility in Bruderheim, Alta.
"However, the customer who cancelled the agreement has indicated they remain interested in discussing a mutually acceptable relationship with Canexus," company president and chief executive Doug Wonnacott said in a statement.
"NATO remains an attractive asset which is evident from the significant interest we have had in the facility since placing it up for sale."
Canexus said the train facility has contracted volumes for up to four unit trains per week, roughly 40 per cent of its planned capacity.
Shares in the company, which were halted pending the announcement, were down 32 cents or nearly eight per cent at $3.85 in trading on the Toronto Stock Exchange on Thursday.
Canexus's main business is the production of chemicals for the pulp and paper and water treatment industries.