<><>Many msgs here have pointed out the need to provide gaps in track rails for expansion/contraction. A number have indicated they fill this gap with a plastic "shim"and glue it in place (then file to shape of the rail).
If the plastic is to be permanent, why not use a plastic insulated rail joiner? Is there more here than I see?
You’re actually confusing two bits of advice:
Expansion joints allow for the movement of the rail to compensate for heat and cold, just like the real thing. Most people recommend using a metal railjoiner here to maintain the rail alignment. A plastic one just isn’t rigid enough.
Insulated rail joints can be planned ahead and a plastic railjoiner can be installed when the track is laid. But some modellers don’t like the plastic joiners (looks, performance, etc) so they cut a gap in the rails to electrically isolate a section. A piece of plastic is inserted into the gap with some epoxy and filed to match the rail contour when hardened. This is to keep the gap from closing up (mechanically and electrically) with rail slide or expansion.
Thx for strenghtening me out on this. The mind suffers when you get old.
Sectional track usually has gaps in the ties under the ends of the rail. This allows the tracks to be joined together without creating a “hump” caused by the thickness of the bottom of the rail joiner. This hump would be far more prominent with the thickness of an insulated rail joiner. It occurs even with metal rail joiners, though not nearly as high. This is why the ties underneath rail joints with metal joiners still have to be shaved down*.* Insulated rail joiners would require a lot more material removal to avoid the hump.