Union Pacific acquires Norfolk Southern, forming the first U.S. transcontinental railroad. Read the latest stock analysis on the merger here.

The largest rail union in the United States said on Tuesday it intends to oppose Union Pacific's proposed $85 billion acquisition of smaller rival Norfolk Southern in regulatory proceedings, citing concerns about how the largest-ever buyout in the sector will affect U.

Union Pacific appears to be using a familiar playbook that large, publicly traded companies trot out when considering mergers and acquisitions: Send up trial balloons and see how they fly. These trial balloons let management gauge market reaction,