By Sabrina Valle (Reuters) -The largest U.S. railroad union said Monday it will support Union Pacific's $85 billion acquisition of Norfolk Southern, helping to advance a deal that surprised competitors and had been expected to face resistance from labor and regulators. The endorsement marks a shift from the initial opposition by SMART-TD, the transportation division of the International Association of Sheet Metal, Air, Rail and Transportation Workers. When the merger was announced in July, the union said ...