You can check occupation on SBB app but really no seat reservations are needed. Trains cannot be fully booked in Switzerland. You can’t make seat reservations on most trains anyway. Even if you make one, people won’t understand that they need to move to let you sit (seat reservations are really not a thing here). Simply board at the train extremities (always less crowded)

Interlaken - Bern - Zurich is not the most scenic journey. I’d recommend to go via Lucerne (if you haven’t done that route obviously). You could also go via Airolo (Old Gotthard Railway), it’s way more scenic than the 57-km Gotthard Tunnel (longest in the world). It add an extra hour. Trains on that route are also less crowded usually.

Again use SBB app (or sbb.ch) for planning. It’ll show you occupation, train compositions (on the day) and eventual delays.

Remember not to take the EC into Como (11€ seat reservation), all other trains are fine.

Let me know for questions

Thank you for you reply in the eurail app Ec 151 doesn't require a seat reservation but I checked Sbb extremely high occupancy for this train what do you recommend please 

Thank 

Please use SBB app for planning. You'll see that due to delays (as it is coming from Germany) it is not stopping in Lucerne today. This one is always crowded (smaller train than usual). I wouldn't recommend to take it, you might not even be allowed to board (really the only Swiss train where it might be the case).

Alternative :

- special train (because EC doesn't stop) Lucerne - Arth-Goldau 12:19 - 12:43

- Arth-Goldau - Bellinzona 12:54 - 15:00

It takes an hour longer but it is a scenic ride on the Old Gotthard Railway (instead of the 57-km tunnel).

From Bellinzona :

- Bellinzona - Chiasso 15:16 - 16:06

- Chiasso - Como S. Giovanni 16:13 - 16:18

Otherwise : you may try to board the EC at Arth-Goldau at 12:49 but staff may deny you boarding (reservation not possible).

Let me know, I'd really recommend the scenic option even if it takes longer.

Btw I will be on the train between Luzern and Interlaken so I'll almost see you hehe ;)