The merger would create the United States’ first coast-to-coast rail network, spanning 50,000 miles and 43 states.

The Surface Transportation Board said on Wednesday that Union Pacific and Norfolk Southern 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.

Union Pacific and Norfolk Southern said on Tuesday that they have agreed to a merger to create a $200 billion rail giant based on their current market capitalizations.