Is this flex track or sectional track? Quite unusual to have this big a gap on sectional track.

well, I presume you have already pulled the rail all the way to one end, so there is only one gap to fill...

Glue something in there, or drop some solder in there. When it has cooled, file off the excess with a hand file.

It does not have to be conductive.

Don't you have on the other side an overhanging piece as well, when you straighten the track? I do not have these flextracks, but the barns on the tie must move a little bit, otherwise you cannot build a curve with this track. But the more you bend it, the bigger has to occur a gap as you show in your picture. How massive did you bend the flex track?

I think this is really a big gap. Both flex tracks connected at this spot must be massivly bended. And the lenght differences between the outer radius from both tracks seems to be added up at this point creating such a big gap.

On the opposite site at this track you see both ends of the bars are a bit longer. Either you shorten them, make less massive curves, change this section against inflexible tracks or do it as it is done in real life at the real tracks. Connecting gaps are regularly welded and then abraded. But @PMPeter mentioned a possibility you may fill up the gap. To fill up the gap by soldering you must try and test it on your own, which result you will achieve. I think @PMPeters solution is good and more flexible as soldering.

Send it back to the engineers, just a drop of hot glue will do the job

Problem is it's already across a joiner and has been ballasted so any repair will do damage to that in which case it might as well be redone.

I agree with the advice to firstly try to gently shift the entire length of short rail to centre it so that there is a smaller gap at both ends rather than such a big gap at one end.

If this is not possible then filling it with solder or simply hot-glue will at least make it less obvious.

Thanks to all of you for the advise!

I can't move the track any more; it's firmly ballasted. I'll try to fill it with solder. If that's not satisfactory I'll resort to epoxy.

Pfalzbahn

I agree. -Also to what applor says; Just leave it!!!

I have these "gaps" in quite a lot of places. They move around with temperature and humidity!!!

If you absolutly HAVE to fill it up, do it with something soft, so that it can at least still be slightly deformed if the track decides to shift again.

Hi

An easy action would be to cut a section of rail of the proper length. Then remove / file the rail "foot" sides so that the rail piece can be inserted vertically in the joiner and glued in place without changing anything in the existing track and ballasting.
Cheers

Jean