What is the Logic behind RAC (Reservation Against Cancellation) Seats in Indian Railways, when there already is a system of waitlisted tickets?

Short Answer:

RAC quota is a kind of "overbooking" used to fill up berths/seats that lie vacant due to no-shows, early exits, quota vacancies and charting errors, to provide a "buffer" between Confirmed and Waitlisted ticket holders and to give some passengers atleast some chance of traveling.

Long Answer:

Yeah, it sounds funny that the railways should have two types of Waiting Lists when only WL should have sufficed. I too have often thought of this, but when you think deeper, it actually looks necessary.

First off, WL and RAC are Quotas. WL has many quotas like RLWL, GNWL, PQWL, RSWL etc, but let us not go into those details for the sake of simplicity. Each train will have a fixed number of RAC quota seats, let us take a certain train has 50 RAC seats for a certain class. Once all the open seats are taken, the next 50 people who book tickets will get RAC tickets and then WL 1, 2 etc will start. If a confirmed ticket holder cancels their ticket, RAC 1 will move on to confirmed and WL 1 will move on to RAC 50 and so on.

Now, as we all know, RAC ticket holders are allowed into the compartment and are provided with seating accommodation, usually two people per berth. WL ticket holders are not even allowed into the compartment and the amount spent by them are automatically refunded into their accounts if booked online.

Now, it is a fact that many berths on many trains lie empty even after them being fully booked. This happens because passengers cancel at the last minute (after chart preparation), get late or simply do not turn up due to a variety of reasons. Berths could lie empty also because some quota or the other did not get filled and RAC/WL tickets did not get allocated to it or due to simple charting mistakes. Now, if RAC quota didn't exist, these berths would stay empty during the run of the train when they could have been allocated to some WL passengers who unfortunately cannot board the train. And this is exactly the reason why RAC exists, so that vacant berths due to passengers not turning up can be allotted to others. Most RAC passengers eventually get berths allotted by the TTE somehow even if the train is full. RAC also helps in reducing corruption and black marketing of no-show vacant berths by TTEs by taking money from WL passengers.

Simply put, RAC is WL passengers who are allowed to board the train. It is a kind of overbooking which enables the train to run full.

Actually speaking, Waiting List is also "Reservation Against Cancellation" since reservation is dependent on cancellation of tickets. Our railways should come up with better naming schemes. "Garib Rath" is another terrible example