So, I looked it up and filcris is HDPE, high density polyethelene.... a number of people here in the US have used it as "ladder track"... usually 2 parallel strips on edge to support the track, and usually then backfilled with earth.
A friend who lives close by here in San Diego, is 15 miles inland of the coast, thus he experiences 20 degrees (F) higher in summer than my coastal location.
The relevance is that before he backfilled the earth around his ladder track, it was very clearly obvious that the HDPE ladder support expanded much more than the plastic track and brass rails. So it seems that not only plastic expands more than the rail, but HDPE even more than the typical G scale plastic ties. (Also most tie strips have gaps between the ties every so often to allow for expansion).
In this case, the HDPE used was also a dark brown, which I'm sure did not help the situation.
Greg