By David Shepardson and Lisa Baertlein WASHINGTON/LOS ANGELES (Reuters) -Workers at the largest U.S. rail union voted against a tentative contract deal reached in September, raising the possibility of a year-end strike that could cause significant damage to the U.S. economy and strand vital shipments of food and fuel. Train and engine service members of the transportation division of the International Association of Sheet Metal, Air, Rail, and Transportation Workers (SMART-TD) narrowly voted to reject the deal. That unit, which includes conductors, brakemen and other workers, joins three other…