CWT is laid in such a way that it has net zero stress in a "neutral" temperature, which naturally depends on the climate. Both extreme heat and extreme cold can cause damage, buckling and fracturing/embrittlement respectively, and choosing the neutral temperature is balancing act. But even if completely cut, track cannot shrink longitudinally much at all, it's the job of the sleepers and the ballast to keep it anchored in place. And if the track is laid on a concrete slab rather than ballast, it isn't moving anywhere.

Fun fact: the reason modern concrete or composite sleepers (e.g. [1]) have a slightly concave profile is to better resist lateral forces (i.e. buckling) than traditional straight-profile wooden sleepers.

[1] https://www.romicgroup.com/permanent-way/concrete-railway-sl...