Montreaux to Zweisimmen -- regular SBB or upgrade to Belle Époque/Panorama

Well the railroad that owns and runs the route is the MOB (Montreux-Oberland Benois), is private railway, so you won't see any "regular 'ol SBB trains" on this route.

One set of trains go all the way from Montreux leaving at 34 past the hour and are dual gauge, so they can transition from the narrow gauge tracks to standard gauge at Zweisimmen. Note on the train says "baggage restricted".

There re a couple of Belle Epoque trains and the rest during day time operaration are panoramric trains, early morning and evening are standard low floor carriages. The Belle Epoque carriages require you step up into them as they are old time cars. All others a low floor carriages.

I'd go with one of the panoramic trains.

OK, now I get it. Thanks. So, as is often the case with these routes, the STP covers the ticket, but seat reservations are additional. I pulled this off one site: "Due to limited seating capacity, reservations are mandatory in the “Prestige” class and recommended for 1st and 2nd class travel, with a surcharge in addition to the valid ticket for the selected travel class." (my emphasis added)

If we already have first class STPs, should we roll the dice on not needing to reserve a seat? I mean, how crowded are these trains, especially if we've sprung for first class STPs? This would be a weekday mid-April, and we're willing to get started first thing in the morning. Is it really a gamble? What happens if you don't have a seat reservation and the car is full? Do you stand? Are you kicked off? Thinking of a worst case scenario where the ride out is fine without reservations; but all the trains returning are fully reserved. Are you just stuck?

I was just watching a video of that route yesterday, search around in Youtube it's pretty popular.

In this guy's case he was on the hot new MOB train which has Premier, first and second class but still had to change at Zwiessman to a BLS train that only had first and second. I believe by now the one trains goes all the way through.

I mean, how crowded are these trains, especially if we've sprung for
first class STPs? This would be a weekday mid-April, and we're willing
to get started first thing in the morning. Is it really a gamble? What
happens if you don't have a seat reservation and the car is full? Do
you stand? Are you kicked off?

I have taken this route often without any reservations (2nd class). Never had to stand. People get on and off all the time - locals, people out for a day hike, etc. - seats open up at every stop. Take one before the people entering at that station get on the train.

No, you will not be kicked off. They can pile a lot of people into a train! Go to the first class car that is furthest away from where most people are getting on, for some reason people don't like walking any distance along the platform for a less busy car, or once on the train ask the conductor where it is less busy, if people are taking seats up with their backpack just ask “is this seat free” and they will move their stuff for you. Lots of times people won't make room unless you ask them to.

Conversely, paying for a reservation is just a fraction of the cost of a holiday, so it’s also not a big thing, in my opinion.

Since you are interested in history, the Belle Epoque might be fun for you. I have only taken it once, but it is a unique train.

Thank you @Maureen. This is really helpful and reassuring. And just to clarify, 100% agree with you that the seat reservation cost is a drop in the bucket on a Switzerland vacation. For us it isn't about avoiding the cost; rather, it is not being locked into one specific train schedule. We want the flexibility to hop on and off; visit stops as the spirit moves us; linger if the weather is lovely, etc. Now I feel more confident we can do just that. Thanks again.