Well, without getting "place to place picky", CSX and NS do have routes in rather close proximity to each other.
This situation has resulted from a lot of subsequent mergers of different railroads over the years, so that the outcome of all the mergers are two large railroad networks that serve many of the same areas of the U.S.
Think of it as a larger fish swallowing a smaller fish, and this "larger swallowing the smaller" process just keeps on going, until there are two very large fish left in the pond. Same parallel with NS and CSX.
NS and CSX are competitors, but when it comes to working out problems together, they have done it when necessary for the common good of both.
Each has given trackage rights to the other, which has meant the savings of a lot of $$, as compared to the ongoing cost of maintaining two sets of individual parallel rail lines.
After the tremendous devastation caused by Hurricane Katrina, NS offered the rerouting of CSX trains over their tracks until the heavily-damaged CSX line along the Gulf Coast could be rebuilt.
Take care and best wishes!
ACLfan2