WASHINGTON — Three of the biggest freight railroads operating in the U.S. have told the government they won’t meet a 2018 deadline to start using safety technology intended to prevent accidents like the deadly derailment of an Amtrak train in Philadelphia last May.Canadian National Railway, CSX Transportation and Norfolk Southern said they won’t be ready until 2020, according to a list provided to The Associated Press by the Federal Railroad Administration.Four commuter railroads — SunRail in …