The New Big 4

A history of railroad mergers at The Big Picture:

and their rail maps:

I am still surprised that Union Pacific and BNSF haven't paired off with CSX and Norfolk Southern.  It wouldn't seem like monopoly if there are two carriers throughout the country.  Maybe it has to do with connections between the systems at St. Louis and Chicago not meshing up well.

Update:  The title referred to this:

The Cleveland, Cincinnati, Chicago and St. Louis Railway, also known as the Big Four Railroad and commonly abbreviated CCC&StL, was a railroad company in the Midwestern United States.
Its primary routes were located in Illinois, Indiana, Michigan, and Ohio.

1918 map with CCC&St. L routes in red. New York Central in orange.