Amtrak is using buses at the Louisiana end of the City of New Orleans' twice daily route and freight trains are delayed because flood damage has closed a train bridge just west of New Orleans.Canadian National Railway spokesman Patrick Waldron says inspectors think debris hit a pier supporting the rail bridge over the Bonnet Carre Spillway. He says the damage was found after an Amtrak engineer reported a rough ride there Sunday.  Amtrak spokesman Marc Magliari says he's been told buses between Hammond and New Orleans will be needed at least until Thursday.  The City of New Orleans runs between Chicago and New Orleans, with stops in Kentucky, Tennessee and Mississippi.  Waldron says about eight freight trains also cross the bridge each day.