since a TTE cannot check the veracity of some other Government(Out of India) I Cards other than Indian, only passports are accepted as a valid ID in train. This is true for us Indians also when we travel outside India - our IDs like Voter, Dr. Lic etc are not valid outside India. Even for Nepal, this ID proof is accepted only in Indian airports and Nepal Govt. accepts the cards, whereas we do not. that is the reason no Nepal Govt. ID are accepted in India.