In lengths like this, without tension, the steel bends easily.
Possibly not what you were inferring in you comment FTN, but it may be useful for others if there was something readily available regarding pre-tensioning of
CWR▸ , which may go some way in explaining the reason behind the speed restriction at Bathampton junction the other week during the warm spell. It was a recently relaid section that may not have been tensioned
IIRC▸ , and possibly not completely tamped.
It's quite interesting how thermal expasion/contraction is compensated for over large sections of solid rail.