We have booked a really cheap flight (less than $100 each, compared to normal $600-800) from Lyon to Schipol Amsterdam in July, and catching a flght back to Australia the next morning on a different airline, and am worried that if the KLM flight is overbooked or cancels for some other reason we will not have any hope of getting to Schipol, is it better to book Fast Train instead?
You may be over thinking this. What time is your KLM flight? You may be able to use rail as an emergency backup if there is a problem with your flight to AMS. There is a train from Lyon airport at 6:41 pm that will get to AMS by 8 am the next day with an overnight in Brussels.
edit-Your flight is being offered by KLM direct. They are a big operator (national airline of Holland) so I don't why you would be worried. You have confirmed seats. Like I say, you have rail back up. You can even make to Amsterdam that night if you leave by 4:29 pm although the connections in Paris look a bit heart-stopping. 2:39 pm would be quite comfortable.
It depends on who you bought the ticket from. If the price seemed too good to be true, you might want to verify that KLM really does have you on its flight list. If so, you are good to go and I second what Sam said. If you aren't on the flight list, your suspicions are confirmed and you have the train times from Sam.
I saw tickets on skyscanner that were so much cheaper than what the airlines were charging that I googled the company selling them to see what others had to say.
Thanks Sam and Bets ,that makes me feel good, thanks very much for your advice. The flight is at 18.15 and confirmed by KLM.
$ 100 is a more reasonable proposition for a Lyon-Schiphol flight than $ 800.
$800 would be something on outrageous territory, I'd think, for such a short flight - unless it is something booked at the last moment.