train from Luzern to Thun - confused on ease

There is no straight line route from Luzern to Thun. That is often the case in Switzerland, the mountains get in the way! The quickest route is via Bern (1h24). However the shorter but slower scenic route is via Interlaken Ost (2h29). This latter route goes over the Brünig pass.
You can look up either route on www.sbb.ch If you just enter Luzern to Thun (and a date and time) it will show the fast route via Bern. If you add "via Interlaken Ost" it will show that route. You can see both routes on the Swiss Railways map here: https://www.swisspasses.com/railpass/overviewmap_en.pdf

Wow very helpful - so if you were on a long haul flight, arriving in Zurich 8:30 am, and thinking going to Luzern for a few hours, would you take the scenic route - I mean is it a must see route? Our second day we planned on going to Grindlewald, and the Murren area.

There is no scenic route Zürich - Luzern. That is 1 hour on the (relatively) flat area north of the Alps. The Alps start after Luzern, you stand on the quayside at Luzern, look out over the lake at a panorama of mountains.
But you said after Luzern you planned to go to Thun, now you say Grindlewald and Mürren. All these are in the "Berner Oberland", south of Luzern. The quick route is round the Alps via Bern, the scenic route is over the Brünig pass. The time penalty is less for Grindlewald and Mürren, because in those cases you haver to go via Interlaken anyway.

I would say if you are travelling from Zürich airport to Grindlewald or Mürren and you have the time, definitely go via Luzern and the Brünig pass route. Should be a nice (and relaxing) introduction to the Alps
See photo here: https://upload.wikimedia.org/wikipedia/commons/a/a7/ZB_Interregio_mit_HGe_101_bei_Niederried.jpg
The alternative is direct Zürich airport direct to Bern and then to Thun, Grindlewald or Mürren. Saves time by keeping away from the Alps.