Todd Sestero's excellent chart showing the history of Railroad Mergers
The Railroads around the perimiter existed for the most part in the 50s and
early 60s. The 7 railroads in blue are the current Class 1 railroads in the U.S   
The Gregarious Railfan
.
12/9/16 UPDATE   Since I created this page, Todd Sestero has
expanded the scope of his chart so here's a
link to the expanded page