I'm a bit puzzled by your reference to "trains from Western France to Hendaye" and changing of trains in Bordeaux. Isn't Bordeaux already pretty much in the west
? Anyway, I have travelled quite a bit on trains in Europe in the last 3 decades and know that I can always get another train if I missed one and that my credit card will provide me with a hotel bed for an unexpected overnight stay but I still like to be well informed when the connection time is short, especially when it is a station I have never been to and with signs and boards in a language I am not overly familiar with.
I've been to Bordeaux main station (called "Saint Jean" station) a few times in the past, and 15 minutes to change trains is eminently doable. Sometimes a train will wait for the arrival of a train that is late, sometimes it won't. The French railways SNCF provide you with up to date info on their websites and apps.
https://www.garesetconnexions.sncf/fr/gares-services has real time info about departure and arrival times and about layout and services at their station.
Below are two screenshots with info for Bordeaux taken from
https://www.garesetconnexions.sncf/fr/gares-services. You see the platform ("voie") assigned to a train and whether a train is on time or delayed. You see how the platforms are arranged, their numbers 1-14, the stairs leading up and down and the tunnels that connect them. One tiny bit of info: the fast trains from and to Bordeaux use the numbered platforms. You may not even have to go down and up, just stay on the platform and change from the right side to the left or vice versa.
Slower regional trains to Hendaye depart sometimes from platform A which puzzled me a bit the first time. You can see the locations of platforms A, B and C on the bottom right corner of the screenshot below (click to enlarge). Bon voyage!
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