Only hours after a wildcat strike, three locomotives were set loose, careened at speeds up to 100 m.p.h. along 13 miles of mountain tracks and plunged into a canyon near Livingston, Mont., injuring a hobo, in what a railroad official called an act of sabotage. Union leaders denied that their members were responsible for the incident early Saturday. A hobo, Allan Eadie, who was riding in the third locomotive, was in stable condition at a Bozeman hospital with fractured ribs, officials said. About 150 United Transportation Union workers staged a wildcat strike Friday night to protest the sale of Burlington Northern tracks to Montana Rail Link. Within two hours, picketers were removed by a restraining order obtained by the railroad.