Since we have been to Paris a few times already, we decided that since we land early in the morning in Paris from Canada, that we are just taking the train to Bordeaux and spending our first night there, enjoy some fine wine and work on our jet lag. The next morning take the two trains to St Jean , have a good nights sleep and on Sept 18........off we go. We've never purchased our train tickets prior to our travels and have never had any issues. We'll probably just buy our tickets in Bordeaux when we arrive for our next day train trips with one of the machines. They are super easy to use and beat standing in long line ups. One trip we took a taxi to Montparnasse station from CDG, what a hoot driving through Paris with an experienced crazy taxi driver. I guess it depends on how fast you need or want to get to St Jean on your decision where to train from.