The 'Head' in HOG does not refer to overhead lines. It refers to the head of the train, which is the locomotive. The original term for using the locomotive's power to directly run the electrical equipment in the coaches is HEP (Head end power). Instead of locally generating power for each coach or carrying a separate coach just for power generation, if the Head of the train, which is the locomotive, itself generates power for the electrical equipment in the coach, then the train is said to have HEP. Locomotives which are capable of this are called HOG capable locos.
WAP5, WAG9 and WDP4 originally came with provisions for adding this functionality, but IR did not...
more... utilize them at that time. Now WAP7 has been enabled for this and a few EMD's as well. WDP4B was initially enabled with HOG for trials. At that time the locos were with TKD. Now WDP4 locos have also been enabled for the same. Recently BGKT DP4's were observed with HOG functionality and were used for the JU JP intercity and probably for the Talgo trials also.