My weird idea would be if sound waves would travel differently down the rail dependent on temperature. Although, the rail is likely not uniform enough to detect the differences?

I found a device that is in use - Verse[1]. It claims to be able to measure the neutral? Although my read says the rail must be below neutral, and therefore under tension, to determine what the neutral is?

They unclip the rail and then lift it with a known force? I guess that lets them measure the tension and lateral forces on the rail?

[1] https://www.pandrol.com/us/product/verse-rail-stress-managem...

https://www.youtube.com/watch?v=Qg9vn50ZOdo

https://www.youtube.com/watch?v=WbNzhMsUOwA

Railway buckling video: https://www.youtube.com/watch?v=jJeiYaSFCf0

Verse PDF: https://railway-news.com/wp-content/uploads/2020/02/VERSE-Te...