Softwood lumber exports from British Columbia to the United States plunged 20 per cent in March from a year ago, the federal government said, amid railway transportation problems.  Global Affairs Canada said in its monthly report that the country's largest lumber-producing province exported 514.4 million board feet to the United States last month, down from 644.7 million board feet a year ago.  "Output from the B.C. Interior remains depressed due to shipping constraints," Hamir Patel of CIBC World Markets wrote in a report.  First-quarter shipments from the B ...