rajkubal, every rail line in Switzerland has at least one train per hour. There is no need to sweat about there being a train, if you go to the station there will be a train.
You can look up train times on the SBB (Swiss Railways) website: https://www.sbb.ch/en/
You can look up, for example, Paris to Luzern or Luzern to Jungfraujoch.
Note, Jungfraujoch takes a lot of time from Luzern, it takes 2h20 from Luzern to Lauterbrunnen (the bottom of the mountain), and a further 1h30 to the top. I would suggest from Luzern you go up Titlis instead, it is closer.
For info about Luzern, start at the official tourist site: https://www.luzern.com/en/
It has a lovely old town, it is on a lake and you can get boats on the lake.
See this 360° panorama of Luzern: http://tour.luzern.com/360/en/