I have 2 layovers in Paris-CDG next month. During the first layover I am staying at the airport, but can I buy 2 RER tickets in advance? I have 4 hours to burn on my way to another country. Coming home, I have an 8 hour layover and I am definitely leaving the airport (I have been to Paris a few times and know where I am going, but I usually take an expensive taxi into Paris central). and I would like to have the RER tickets in hand already to save time. I remember that as long as I don't put them in the machine, I can use them a week later? I just need a Paris train refresher course!