The IRCTC, which runs the online ticketing engine for the Indian Railways, and Air India have recently entered into an agreement, where if your Rajdhani ticket does not confirm (is waitlisted) till the time of departure of the train, you can get a ticket on an Air India flight within the next 24 hours.
Essentially, everyone with a waitlisted ticket on a Rajdhani Express, with effect from June 1, 2016, will be offered an option by IRCTC to travel on an AI flight if their train ticket is not confirmed by the time of departure. A confirmation needs to go back to the airline within the next 24 hours for the offer to be valid.
First Class passengers will get the ticket at the same price as their First Class ticket (which is, in some cases, more expensive than an Economy Class flight ticket anyways!). Passengers in the ACII and ACIII compartments will have to pay another INR 1500 to 2000 to get the flight ticket.
Does this make sense?
On trunk routes, maybe not. For instance, if you can afford to pay for first class on Rajdhani, you have paid more than a flight ticket on the route already, that too a last minute ticket. Have a look at the Rajdhani First Class fare between Mumbai and Delhi.
Here is a look at flight prices on the same day at INR 4462. If you want to travel the next morning, it is actually INR cheaper than the train.
I did the exercise for a couple of other Rajdhani Express routes, and at least on metro routes, it made no sense to keep your train ticket for your flight. You could just collect a full refund and take a flight on the cheap anyways.
Unless you live in a place where train connectivity is the only option, it usually makes more sense to take a flight anyways. Also, flight tickets are cheaper on most routes even for last day pricing since this is not the season for travel around India anyways. So, measure up the pricing before you take the train!