BILLINGS, Mont. — The federal government predicts that trains hauling crude oil or ethanol will derail an average of 10 times a year over the next two decades, causing more than $4 billion in damage and possibly killing hundreds of people if an accident happens in a densely populated part of the U.S. The projection comes from a previously unreported analysis by the Department of Transportation that reviewed the risks of moving vast quantities of both fuels across the nation and through major cities. The study …