Train travel. Switzerland to france

It will go something like this and take about 5 hours:
Train Lauterbrunnen to Interlaken Ost
Change
Train Interlaken to Basel
Change
Train Basel to Strasbourg
Change
Train
Strasbourg to Saverne

In Basel, the French trains leave on different tracks than the Swiss trains. When I was there precovid, one went outside the main train station entrance, and took a left turn to the "French" side of the Basel station. Perhaps someone can tell if it's still the same. Safe travels!

Download the SBB app. You’ll need it while in Switzerland anyway. Plug in your start and end point and the various options will appear, including travel time, number of connections, etc. just be aware that the price you see assumes you have (or will have) the Half Fare Card. If you do not, you need to double the price.

You can't buy international tickets via the SBB app. You can however do so on the website.

You can also use Trainline, which is often cheaper as it will split the ticket at the border and try to find the cheapest ticket for each segment.

In Basel it was indeed the case that you had to go outside and use the sidewalk to get to the French sector, as the corridor between the Swiss sectors was being renovated. Those works have however now finished, and you can again walk to the French sector within station.

The result of the renovation is quite nice. The corridor now looks again as it was originally, with all the ugly stuff that had been added since removed again. There is a big Migros supermarket, and a take-away in case you want to pick up something to eat on the train, and towards the end, near the French sector there is now a nice bistro in case you have a longer wait.