Rail fractures arise with change in temperature..which lead to shrink/expansion of tracks, mostly in winter when huge difference exist in between day & night...(with track been galvanized @ joints & only station loop/track merge/diverge break-even tracks shrink/expansion, as they remains free from galvanized joint...
Shrink/expand also lead in track dishape, with bend & curve, which also been checked by trolley run or single tyre unit....
Gangmen and trolley unit do the routine check regularly....while there exist work cut-off with minimum 8 hrs in single track while 6 hrs in double track!! Mumbai...
more... Local terrain been checked in late night hrs...have seen gangmen active @ 2AM even in single line traction!!! When issue identifies, caution been kmplemented & necessary work been done later...