How do you select and board a Swiss train?

Yes. The Swiss Travel Pass is an "all you can eat" pass. You can just use any public transport at your own discretion.

Swiss public transit is entirely based on "proof of payment". In order to travel you need to be able to prove upon request that you have indeed paid for the privilege to travel. Your STP is your proof of payment. There are no ticket barriers, no pre-boarding procedures. You usually can board trams and busses through any door and do not have to pass via the driver.

You will notice that in many stations you can go directly from the street to the platform.

You also do not need to select anything in the SBB app. The app is just for convenience, to help you find your train. It helps you be on the right train, but is not essential. So just board the train, bus, or tram you want to take, find a seat and relax.

Thanks so much for ur help

Note that you do not need reservations for the Golden Pass route either. The Luzern - Interlaken train is just a normal service that is used by locals to go about their daily business. It just happens to have big windows and scenery. You can reserve seats on that train but I would not bother.
For the Interlaken - Montreux GPX trains I would get a reservation though. But you can get that here at the station.

You may be asked for ID when using the Swiss Travel Pass. So, be sure to have your passport ready to show if asked.

And you only have to show the Pass if asked. They don’t check it on all the trains.

I like to also have a paper copy of the Pass. I find it easier to use then trying to pull up the Pass on my phone while the conductor is standing there waiting.

Do they check for first class passes? We are thinking about going first class from Luzern to Interlaken and upgrading for that part of the trip.
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I believe they do. You board the train and show ur tix or 1st class Swiss pass to the conductor on board.

in terms of how to board - go to the door of the car your reservation is in, and when the doors are released open the door and tep on, find the number near the seat you have reserved and sit down.

If you don't have a reservation go to an open car with a big number on the door - a 1 for a first class ticket or pass, or a 2 for a second class ticket or pass. Step on and find a seat which does not have a reservation noted and sit down.

Do they check for first class passes?

Yes! You will need to show your first class upgrade along with your STP, or whatever you have that qualifies as a 2nd class ticket for the journey.