The deal, which would build the first coast-to-coast freight rail, could take up to two years to complete amid scrutiny.
Union Pacific has announced its intentions to buy its smaller rival, Norfolk Southern, which would create the first coast-to-coast freight rail operator in the United States and reshape the movement of goods from grains to autos across the US.
The Omaha, Nebraska-based railroad giant announced the proposed $85bn deal on Tuesday.
If the merger is approved, the transaction would be the largest-ever buyout in the railroad sector.
Union Pacific has a stronghold in the western two-thirds of the US, with Norfolk’s 31,382 km (19,500-mile) network that primarily spans 22 eastern states.
