^Don't know why they wouldn't. It's not like that JAXPORT line is more important to them than the mainline they sold to state for Sunrail ($150 million for 61 miles of track). This is America, everything is for sale (Ask Winn-Dixie). It's just a matter of coming to the table with a deal they'd consider. However, you'll never know if you don't present them with an offer or make a serious inquiry. As much money as JAXPORT wants to expand, the cost of purchasing track from CSX would amount to a pimple on the ass of the overall picture. However, the benefit to the companies the port wants to attract would be significant.
Btw, CSX has already turned over half the line to St. Mary's Railroad and First Coast Railroad. Their business model isn't pulling single box cars into warehouse sidings these days. So precedence has been set. Nevertheless, if they don't play, COJ already owns the S-Line and rebuilding it would at least provide FEC with access to Talleyrand.