To take advantage of the best prices you will need to prove you are not a business traveller, on the way to the airport, so you will need to give either yr flight numbers or your holiday booking reference. I don't think you will need to provide this when you arrive at caledonian-road station but you may be asked to provide them on your travels from the station to the airport.
Going to stansted airport on the train is great, you can take a nap, read a book (even at night), play a game, or just gaze out of the window... The other restriction to buying these cheap train tickets from the station to stansted airport is that there must be at least two grown ups travelling together. But another great advantage is that children go free. All children under 5 go free, and then one child over five but under 16 per grown up travelling on the 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-road station before you go. If you end up with 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-road station.
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