WASHINGTON (Reuters) -The Surface Transportation Board said on Wednesday that Union Pacific ( UNP ) and Norfolk Southern ( NSC ) have formally filed a notice of intent to combine, triggering the U.S. regulatory review process for a proposed $85 billion merger between the two major railroads.

The STB said the railroads intend to file their application by January 29. Once filed, the STB will consider the application and set a schedule for the merger review that could take a total of 12 to 18 months.