Most trains can be booked 91 days in advance, and some short distance trains (daytime trains) can only be booked 30 days in advance, there are a few trains with even shorter booking periods, but none on the main tourist routes. In case one delays in booking a train ticket, one has to follow certain procedures:
Process Flow:
When all the available seats/berths in an individual train/class are sold, the railways start selling RAC tickets (2A, 3A, FC & SL only). When all RAC tickets have been sold, then a waitlist begins to be prepared for all the classes in the train, and you are then sold a waitlisted ticket.
As people cancel tickets, an RAC/WL ticket will move closer to a confirmed berth. Also, when the final reservation chart is prepared, any unsold quota berths will be used to reduce the RAC/waitlist.