Union Pacific and Norfolk Southern are confirming they're in merger talks that would create North America’s largest railroad, stretching service from East to West Coasts.

The combination of two large rail companies would create the first coast-to-coast network, but the deal could reduce competition.

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%.