Horwath says the contract should have gone to Ontario Northland, a company with a production plant in North Bay that has fixed up the trains for more than five years.
She says the loss of the contract has put more than 100 jobs at risk.
Horwath made the pledge while talking up her party's Buy Ontario program during a campaign stop at the North Bay plant.
Metrolinx, a provincial agency in charge of the GO refurbishment deal, recently announced the next five years of work would go to Cad Railway Industries near Montreal.
The NDP says it believes the contract hasn't been signed yet and there wouldn't be penalties for cancelling it.
If the contract has been finalized, an NDP government would look into the costs of ripping it up, the party says.
Horwath also stopped by Temagami provincial park Saturday to announce her party's tourism platform.
The program would make day access to the parks free and cost the province $7 million a year.