A 48-car runaway freight train carrying chemicals slammed into a parked engine Thursday, setting off an explosion that blasted out windows up to 3 miles away and cut power to the Helena, Mont., area, officials said. Dispatchers said no injuries were reported, although damage was heavy, particularly to Carroll College where 1,100 students were evacuated for fear of another explosion. A spokeswoman for Burlington Northern Railway said crews were switching engines when the cars got away. A Montana Rail Link spokeswoman said engineers set the air brakes on the cars but they failed, possibly because of the intense cold, and the cars broke loose.