Sacrifice the tie. I would take a pair of needle-nose pliers and brute-force it out, starting with the outside pieces. It's easier with flex-track, because the ties are only tied to their neighbors on one side. After the outside parts are gone, then do the same for the inside. Take a tie from another piece of track and cut off all the plate and spike stuff, so you just have a rectangular box. Once the joiner is on, slip this reduced tie underneath the track to fill the gap.
It takes an iron man to play with a toy iron horse.
Feed a coping-saw blade under the tracks and gently cut the plastic webs that connect the offending tie to its neighbours. Then if you can, pull out the tie and shave it where the rails go; if it isn't free to move, simply destroy it with the same sawblade, and replace it with another that has been shaved. You may need to remove the pin that goes thru the blade at the end to be inserted.
Have fun, George.
Go in from the edges, at right angles to the rails, with a chisel-point hobby blade. That takes off the fake spikes and tie plates and leaves clearance under the rail so the joiner won't hump it up.
Just did that myself a few minutes ago, on the non-sliding rail of a piece of Atlas code 100 flex destined for use in a cassette. The cassette is 54 inches long, but the flex is only 36.
Chuck (modeling Central Japan in September, 1964)