Norfolk stock traded as high as $288.11, but closed at $278, down 0.8%, while Union Pacific stock lost 4.5%, closing at $220.52. The S&P 500 added 0.1% and the Dow Jones Industrial Average lost 0.7%.

Union Pacific's adjusted profit grew to $1.8 billion in the second quarter as merger rumors to swirl around the nation's largest railroad.

Union Pacific Corp. (UP) and Norfolk Southern Corp., major grain shippers and two of the six remaining Class I freight railroads in North America, confirmed they are in “advanced discussions” regarding a potential merger.