(1) Actual reason can be known only after the detailed investigation by experts is over.
(2) Current temperatures are not too low our rails are working well at sub zero temperatures in Kashmir Valley.
(3) The age of the rail need to be taken into account
(4) The impact of the damaged wheel or the derailment itself may cause rail to break. ...
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(5) After derailment many things rail, sleepers etc may be found broken, but what was the status before derailment can be judged by detailed investigation of the last 1 or 2 km of track crossed by the train before derailment and the inspection of vital coaches and its components like bogeys etc.
So many possibilities are there
(A) At first instance, politicians have tendency to point fingers at opponents, IR officials on the equipments supplied by the other organisations and Media/general public on RM/IR employees. Let the final report of investigation come out to know the real reason.
(B) Political expediency, financial imprudence, inherent inefficiencies in working/decision making and poor change management practiced over decades is the root cause for most of the maladies in IR today.
(C) Any body may be found responsible for this accident, but not much can be done to ensure safety unless the overall system improvement is done.