While in some cases, the alphabets relate to the name given to the station by the Britishers, in other cases it is for the convenience of... read full article... C for Kurla: Decoding Mumbai railway station codes in
Regarding X - the theory of HX seems right. Take for example XBKJ and BKJ. XBKJ was built before BKJ which is in KR. So why use X in Bikramganj? It must be due to the marking of stations with X on map and its subsequent use in station code.
AFAIK , Heranyanagaram was the old name for Cuddapah/Kadapa , it was a major crossing station apparently , thus it was referred as HX. This was what I got to know from veterans
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