I don't know for sure, but based on ride quality, I would guess most of Central Florida...
No argument with you regarding ride quality in general on the Amtrak trains at speeds over 60mph in Central Fl. But almost all of CSX mainline trackage South of Jacksonville is continuous welded rail. Now sidings, yard, industry leads, etc are a different story.
Both main tracks between Beaver St and St Johns (in JAX area on the A-line) were jointed rail for the longest time though I think it is just a small portion of the #2 main track these days.
Track #1 on the A-line between North end double track WPK and South end double track ORL is still jointed rail, but the trains are traveling roughly thirty miles per hour max in this section.
The main track between TN crossover (A-line in TPA where Clearwater Sub and Tampa Terminal Sub cross) in Ybor City and the trolley crossing is still jointed rail. You'll notice it more (they'll be traveling faster) when the train is pulling out of TPA rather than when backing into TPA.
On the S-line in between Wildwood and Coleman (where the Palmetto used to run now replaced with a bus service), track #2 is still jointed rail and a passenger train can operate at maximum authorized speed of 79mph on that section as well. You really notice it here!
I am not sure, but there was still one small section of jointed rail on the S-line between United and Dyer (between WPB and St Lucie Canal drawbridge), and a passenger train could operate at max authorized speed of 79mph. That section used to be really , really bad! I think this little CSX portion has been replaced with welded rail now. It used to be jointed rail from United all the way into WPB depot, but the state changed their portion to continuous welded rail after they bought it.
and most CSX lines in general for that matter!
Pretty much after you get North of Savannah on the A-line, almost all of the long sidings (which would be classified as double track) are still jointed rail almost all the way between there and Collier Yard (South of PTB in VA) with the exception of around FLO itself.
On the S-line North of SAV(Silver Star's route between SAV and RGH), there is a section of jointed rail on the Columbia Sub between Fairfax, SC and DNK. The Star operates at max authorized speed of 79mph over it.
Between Norway, SC and Cayce Yard in CLB there are multiple sections of jointed rail. There are a lot of curves with speed restrictions here but the max authorized passenger speed is 60mph I think.
And finally on the Aberdeen Sub between around Aberdeen, NC and Sanford, NC there is a really long stretch of jointed rail. The max authorized passenger speed is 60mph over this, but there are speed restrictions for curves and town ordinances.
Outside of those areas I am not qualified (actually my Carolina qualifications have expired now since it has been over a year), so I do not know the details of the rest of CSX.
OBS gone freight...