Union Pacific's (UNP) proposed merger with Norfolk Southern (NSC) was sent back by the US Surface Transportation Board, saying that the application was "incomplete," the regulator said Friday.

The board said the December filing lacked required information, including projections of future market share and the merger's impact on competition.

While the railroad companies provided 2023 market-share data, the Surface Transportation Board said they did not include future market projections that showed expected traffic growth, diversions and other market changes resulting from the merger.

"Under the law, the Board therefore must reject the application, and does so without prejudice to Applicants refiling a revised application remedying the deficiencies identified in the decision," the regulator said.