There is one scheduled daily train service that runs from Paris Montparnasse to Lisbon.
This is the Sud-Expresso.
In reality, it is two trains:
TGV - Paris to Irun
Normal Overnight Trains - Irun to Lisbon Oriente.
Timetable here: http://bit.ly/duWas1
Lisbon Oriente to the Algarve
Although the North-Bound TGV stops in Poitier and Angouelme, southbound it does not.
Therefore, by train it does mean going up to Paris and then down again.
See also this articel on my blog: http://bit.ly/bogRy6
It's a real pain. We have a friend who stays near CarHire-Plougaix and we have looked at alternative means of travelling from the Algarve to Brittany to see her. In then end, its worked out cheaper to drive even though its a round trip of over 3000kms.
BTW ryanair.com and easyjet.com have recently started operating flights from the Paris region to Faro. In winter, only ryanair.com will operate twice weekly from Paris Beauvais to Faro.
