Rockfort Express: திருச்சி அவனது கோட்டை, இரவில் சென்னை வரை வேட்டை, அவன் தான் மலைக்கோட்டை. - Vijay Baradwaj

Thank you all for your very interesting observations and comments.
I am sorry the discussion veered round to some other topic than the original one. As for repeated questions, people like me who dont visit this site often, are a bit awed while searching for similar questions asked earlier. Further, there is a possiblity that rules might have changed since the question was asked earlier and so a repeat would perhaps bring out the latest development in the matter. All the same, I thank everyone who participated here.
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all who responded, I would like to mention the following:
a) An e mail reply from irctc.co.in (asking for clarification on the same issue) is quoted below:
"Kindly note that it is not possible to board the train from the originating station if you do not have the ticket from the originating station. First you have to buy the ticket from the originating station prior to the journey and then you can board the train from there. "
b) When I enquired at a Railway ticket reservation counter, the person at the counter said that the passenger has to contact the TTE at CST, explain the situation, pay the fare between CST and Dadar to him and obtain a receipt. He can then travel on the same seat for which he had booked the ticket from Dadar, since there is no halt between CST and Dadar. This gentlemen also mentioned, for example, that the fare payable between CST and Dadar by this train for an AC 2-Tier seat is Rs. 120/-.
Maybe this matter is still not clarifed to the satisfaction of all!
I would once again mention that the passenger in question is not interested in travelling free between CST and Dadar, and is willing to pay the difference in fare, so that he could travel legally. His intention is clear on this aspect, as mentioned in the original questio,
All the same, thanks to all of you once again. Regards.