I can think of three reasons it won't happen.
First, after some years of instability, there's been a sort of homeostasis within the rail industry. After all the scrambling of who was going to get the odd players out - the D&RGW; the SP; Conrail...now the game is pretty neatly divided four ways. And some of those four players haven't yet learned how to "move their pieces" to best effect.
That's theory. In practice...the CSX physical plant is run-down and getting worse. Any rail company that buys it out, is going to have to make up all that deferred maintenance. This is daunting to the point of working against the merger...if, say, the UP bought CSX, and then had to raise capital to renovate it...the UP itself could become a target for a larger company, a conglomerate or even a takeover specialist. It would be severely weakened.
Besides, with the property so bad...why buy it when odds are you could get it for nothing from Bankruptcy Court in a few years? The government may even PAY someone to take it over, not unlike what is done with failed banks.
Third reason...in this environment, with the short-term future uncertain, most businesses are gong to hunker down and not take risks. Including taking the risk of buying a struggling competitor.
There are other reasons, but those are the biggies.