The move comes as part of a merger where Union Pacific will be acquiring Norfolk Southern, and together they'll form a company worth of over $250 billion.

Union Pacific said on Tuesday it would buy smaller rival Norfolk Southern in an $85 billion deal to create the first U.S. coast-to-coast freight rail operator and reshape the movement of goods from grains to autos across the country.

Landmark mergers in the past - such as the creation of BNSF and between Union Pacific and Southern Pacific - have set the stage for today's concentrated rail network, often triggering both operational gains and regulatory pushback. Here are some of the largest railroad deals in the past three decades: