My solution is to glue styrene strips to the inside edge of the guard rail to narrow the gap, then paint it to match. Others fill the gap with that blue epoxy putty stuff that feels like clay when mixed, then run a car back on forth by hand to create a flange way until it sets up. Still others trim the existing guard rail completely out, then fabricate a new one. And there are still other ways of dealing with it.
I think I'm having the same problem with my turnouts.
Can you elaborate a bit on exactly where you put the styrene? Are you talking about putting it against the black plastic guard rail on the outer edges of the track?
Thanks.