I need to get from Paris CDG to Bordeaux and there appears to be a direct train from the airport, but it seems to be full. Is going to Montparnasse a viable option to catch another train?
You can either take the direct train which is a little slower (given it goes around Paris at slower speeds) or go to Montparnasse where the high-speed services run from. You will have a lot more services from there but you have to deal with the hassle of going to Montparnasse. There is a direct bus service from CDG to Montparnasse as well as local rail (RER+transfer on the Metro). It'll take 1 hour at least to go from CDG to Montparnasse. I would be tempted to either (a) try to grab a train from CDG (more convenient) or fly from CDG to BOD. It could even be cheaper.