To take advantage of the best prices you will need to prove you are not a business traveller, en route to the airport, so you will need to provide either yr flight details or your holiday booking reference. I don't think you will need to provide this when you arrive at farnborough-north station but you may be asked to provide them on your travels from farnborough-north station to the airport.
Going to the airport on the 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 cheap train tickets from farnborough-north station to heathrow 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 adult travelling by train to heathrow airport.
be sure to leave enough time to get to the airport early enough to eat, shop and so on, so check the times of trains from farnborough-north station before you go. If you end up with time to kill at heathrow airport you can always read a book or play travel scrabble again! As you have bought your train ticket in advance, you don't need to worry about the opening times of the station.
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