Union Pacific and Norfolk Southern are in merger talks to form the largest railroad in North America, connecting the East and West Coasts.

The deal would unite the largest and smallest of the six major U.S. freight railroads.

While both companies declined to comment, approval from the Surface Transportation Board (STB) could be challenging due to strict merger rules set in 2001.

Proponents say the merger would streamline shipping, but critics warn it could reduce competition, the Associated Press has reported.

The STB’s current split makeup and public scrutiny mean any deal would face a lengthy and difficult review.

Written by B.C. Begley