Oho!! IR Rule:
1. Mail/Exp: U need unreserved Ticket. When train arrives, u can ask TTE as per reserved class choice & Availability. In case AVL, TTE will collect Fare Difference & issue receipt & assign vacant berth!!
2. SF/Janshatabdi/Garib Rath: Same as Pt. 1 but with Unreserved SF Ticket!!
3. Rajdhani/Duronto/Shatabdi AC: 1st class unreserved Ticket. On train arrival, same process as Pt....
more... 1!! In case its not 1st Class Unreserved Ticket, Fine will be levied along with Difference Fare!!
PS: Unreserved SL Class tciket can be purchased to board SL Class for several train, in case berth available for travel stretch!!