To take benefit of the best prices you will need to prove you are a leisure traveller, on the way to the airport, so you will need to provide either your flight numbers or your holiday booking reference. I don't think you will need to give this when you arrive at the station but you may be asked to give them on your journey from caledonian station to the airport.
Going to the airport on the train is great, you can sleep, read (even at night), play travel scrabble, or just stare out of the window... The other caveat to buying these cheap train tickets from caledonian station to stansted airport is that there must be at least two grown ups travelling together. But another great advantage is that kids go free. All children under 5 go free, and then one child over five but under sixteen per adult travelling by train to the airport.
be sure to leave plenty of time to get to the airport early enough to eat, shop etc, so check the times of trains from caledonian station before you go. If you have time to kill at the airport you can always read or play travel scrabble again! As you have bought yr train ticket in advance, you don't need to worry about the opening times of caledonian station.
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