All the Durantos. you cannot book the ticket from an intermediate station, granted. You may deboard at any Technical halt, that granted too. The million dollar question is can you board at an intermediate station. For example I am traveling HWH-CSTM Duranto, can I purchase a ticket HWH_CSTM, booking from HWH, boarding at TATA? In that case the train TTE will know, even if a particular berth is vacant at HWH, the passenger will board later, and that the particular berth is not to ba allocated to any RAC passenger.
We can do that in any other trains. Specially if the quota at your station is full you may book from a previous station, pay from that station for your journey,...
more... and still you may board at your convenient station, and your seat still remains reserved.
The rule is you cannot book a ticket, it's quite mum about boarding. I would say since I have paid for the full journey, railways should not mind where I board from, as long as I have indicated that, at the time of my booking?