Well, if you're asking about flexible tubular track, of a style compatible with Lionel, I have no idea! Never seen it, to the best of my 67 years knowledge.
On the other hand, you can get Gargraves flexible track to marry up to tubular if you buy Gargraves' adapter pins which connect the rectangular end of their rails to the round end of Lionel's.
HOWEVER!!!!....
If you have never tried bending Gargraves flex track into a decent smooth curve, you're in for (how can I say this?)...a 'learning experience'. It ain't easy for the novice. (It ain't easy for those more experienced, either?) AND...you will have to re-cut the rail ends after bending to square them up for mating to the Lionel track.
Atlas also makes a flex track, and I'm certain it can marry up to Lionel with the correct joiners. I suspect it might be easier to bend into a smooth, consistent curve. However, here again, you'll have to re-cut the ends of the flex track to square them up to the Lionel.
All-in-all, it would be easier to redo tables to accommodate fixed curve choices among the manufacturers...methinks.
Just trying to save you from a migraine....
KD