At Electro-Motive Diesel, nearly 500 unionized workers have been locked out of the London plant after refusing to accept a contract offer that would cut wages in half, eliminate pensions and cut down other benefits.
Members of the Canadian Auto Workers union set up picket lines at the plant Sunday evening and continue to picket into Monday.
The company is owned by Caterpillar, a heavy equipment giant known to play hardball with unions.
CAW members say they're worried the company will move production to a plant in Indiana.
Plant chair Bob Scott says workers will stay out until the company agrees to return to the bargaining table.
The collective agreement between the plant and union ran out at the end of 2011.