To take advantage of these prices you will need to prove you are a leisure traveller, on the way to the airport, so you will need to give either your flight details 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 provide them on your journey from the station to the airport.
Going to stansted airport by train is great, you can sleep, read a book (even at night), play travel scrabble, or just stare out of the window... The other restriction to buying these inexpensive train tickets from the station to the airport is that there must be at least two adults travelling together. But another great advantage is that children go free. All children under five go free, and then one child over five but under 16 per grown up travelling by train to the airport.
make sure you leave enough time to get to stansted airport early enough to eat, shop etc, so check the times of trains from caledonian station before you go. If you have spare time at stansted airport you can always read or play a game again! As you have bought your train ticket in advance, you don't need to fret about the opening hours of the station.
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