Hopefully Bernard who is really knowledgeable about the whole area will come along. I usually have a car when traveling so I might forget some possibilities by public transport. But over the years, here are the information I was able to gather.
Along the train line from Nantes to Le Croisic serves many sea resorts : St Nazaire, Pornichet, La Baule, Le Pouliguen, Batz sur Mer and Le Croisic.
You will find good beaches east of Pornichet. In Batz-sur-mer and le Croisic are probably the best beaches. Those beaches are at about 10 minutes walking from the railway station. The best beaches are in Batz sur mer and le Croisic.
Beside sandy beaches le Croisic and Pornic also offer scenic fishing harbour.
Near St Nazaire the beaches are not near the railway station (about 30-40 minutes walk) but you will find a few different options : Villes Martin, Bonne Anse, plage de la Courance, St-Marc-sur-mer. https://www.saint-nazaire-tourisme.uk/explore/discover-our-coast/saint-nazaires-seafront/
In St-Nazaire, le Pouliguen and Le Croisic, it's possible to hire bikes. There is a cycling path named "vélocean" that you can take to explore the area.
La Baule has a very long beach which is nice to have. But it's a more modern resort with lots of concrete buildings. Still many people go there every year because they like the area but just so you know.
https://www.levoyageanantes.fr/en/by-train-around-nantes/
https://www.tourisme-lecroisic.fr/en/
https://en.pornic.com/
https://www.brittanytourism.com/destinations/the-10-destinations/brittany-loire-ocean/pornic-and-the-retz-area/
https://www.ot-batzsurmer.fr/
https://en.labaule-guerande.com/la-baule-guerande-peninsula-and-its-surroundings.html
https://www.atlantic-loire-valley.com/atlantic-coast
https://www.levoyageanantes.fr/en/to-do/
The public transport website for this area : https://aleop.paysdelaloire.fr/
The TER sncf website offers some deals that might interest you : https://www.ter.sncf.com/pays-de-la-loire/ete-aleop-en-ter
https://www.ter.sncf.com/pays-de-la-loire/decouvrir/balades-plages-en-pays-de-la-loire
Edited: 16 August 2022, 10:26