Adds shares in paragraphs 2, details from the release in paragraph 3 and 4, background in paragraph 5

May 28 (Reuters) - The U.S. Surface Transportation Board said on Thursday it has paused the review of railroad operators Union Pacific UNP.N and Norfolk Southern's NSC.N merger application, while it seeks more information from the companies.

Shares of Union Pacific fell 4.2% in morning trading, while those of Norfolk were down 5.4%.

The companies' merger application has been accepted, but proceedings, including the environmental review, have been put on hold, the STB said.

"Several aspects of the revised application... are unclear or underdeveloped and require supplementation at this stage," it added.

The companies filed a revised application in April for their $85 billion merger to create a coast-to-coast U.S. freight rail operator after an initial proposal in January was sent back.