Failed sc & st candidates reservation

you asked

"Under which Constitutional Right OR Article are these eligible even without these passing with minimum marks atleast with reexam.Kindly convey the rules as we can see the output of such candidate has been affecting the functioning of Railways drastically."

The tone and tenor indicates that you are as a superior officer seeking explanation of the subordinate.

Unfortunately the experts providing charitable service here are not accountable to you. 

Also the experts here are not spokespersons of the Railways.

You can seek such clarification from Railways in polite tone and if necessary seek copy of relevant noting under RTI.