Union Pacific Corp. and Norfolk Southern Corp. shares plunged more than a year after the US Surface Transportation Board paused its review of their planned $72 billion merger, which would be the largest railroad deal ever. The regulatory setback sent both stocks tumbling as investors reconsidered the deal's timeline and prospects.

The board accepted the application but put further considerations on hold until companies submit additional information by July 27. Regulators specifically requested clarification on matters "that are unclear or underdeveloped and require supplementation," creating uncertainty about the merger's future progress.

The delay significantly complicates what was positioned as a transformative industry consolidation. With regulatory scrutiny intensifying and antitrust concerns likely playing a role, investors now face a longer timeline for potential synergies and cost savings from the unprecedented combination.