I just learned this week that, "Long Hood Running" was due to RR's needing to have the one person crew (engineer only) to sit on the right side of the long hood, and that he be able to read trackside signals, that are on the right side of the rail line (especially in switching operations and coal service runs).
I've read a number of theories on this topic, and I've never heard that one before. I assume you are referring to North America? I have equated "Long hood Running" with the long-hood forward running, or what appears to be a diesel engine running backwards. Forgive me if I misunderstood.
One theory is that early diesel manufacturers designed the first generation locomotives with cabs at the rear to mimic the shape of steam locomotives, therefore providing engine crews some familiarity with the operating environment and some comfort in being protected in the event of a front-end collision.
Going against your statement about one-person crews, running a locomotive long-hood forward actually hindered the engineer's ability to operate alone, with nearly no visibility around left-hand curves. There was some unrest at that time within the engineers union about the recent lack of necessity of the fireman. Producing new road-switchers with limited visibility probably helped secure the fireman's job for years. This way of thinking changed by the late 1950's, and, due to crew safety (visibility), short hood diesels began to become the standard.
The N&W and Southern Railway both ordered locomotives with high short hoods, designated long-hood forward, well into the 1970's. SOU units were purely long hood forward, while N&W units commonly had dual controls so the engineer could sit on either side. This practice of running long-hood forward eventually faded, and several Class 1 railroads have a rule that states if turning facilities are available a train may not leave the terminal with the long hood forward.
I have run both high-hood geeps and an RS-3 and the visibility issue is very apparent. EMD switchers are not as bad, because you sit a little higher than the top of the hood.
Back to the topic: What two railroads merged in 1980 to form CSX? I'll give it a go, without any fact checking to keep it fair. Be warned, the information below could be complete BS. I'm going off the top of my head...
The first thing that comes to mind are the Chessie System and Seaboard System, but that happened a few years later, IIRC. The Seaboard didn't gather the Family Lines together til 1982, I think. The Chessie System was also a loosely connected group of railroads that chose to cooperate rather than merge in the wake of the merger difficulties in the 1960's. The C&O was the dominant party I believe. I think they finally combined all the railroads (B&O, C&O, WM) around 1980. My guess is the Baltimore & Ohio and Chesapeake & Ohio...final answer.
Edit: I got it wrong...

The overall idea is correct, but there is a detail in the merger that better suits the question. Good luck!
Tyler