I have travelled many times in SL/AC with unreserved Ticket with paying the difference & geting Receipt for the same!!!
In TAAG also, there is discussion about provision to board train with unreserved ticket along with restriction & chargeable distances!!!
Everytime, u will find travellers discussing about vacancy with TTE, with unreserved ticket in-hand
Even, people board Rajdhani with first class unreserved ticket from...
more... CNB & get difference paid (I have done this for Rajdhani & Shatabdi)
U have the option to pay extra (if vacancy) or else get fined!!!
That is what, u get to knw, when u speak with TTE!!!!