CSX may also be responding to activist pressure, the TD Cowen analysts said. Ancora Holdings has been pushing CSX to either pursue a deal with a rival or replace its chief executive, The Wall Street Journal reported this month.

A partnership between CSX Corp. (CSX) and Berkshire Hathaways' (BRK.B) BNSF isn't a replacement for an eventual combination of the two railroads, according to an Evercore ISI analyst.