You already have some great suggestions and advice from Lucas and Slowpoke. I have a few suggestions and comments for you to consider as well.
>> From there, my wife and I will proceed using trains to Interlaken (4 nights), Zermatt (3 nights) and Montreux (3 nights) and than back to Zurich for our flight home. … Does that itinerary make sense (in terms of order
As Lucas mentioned, visiting these towns in the order proposed does not really make the most of opportunities for scenic transfers from town to town. I note your preference to visit the Jungfrau region first, so what about doing things in this order:
Zürich-Interlaken - Golden Pass scenic route via Luzern (not via Bern)
Interlaken-Montreux - Golden Pass scenic route (second leg)
Apart from the sector from Visp to Zermatt, which is part of the Glacier Express route, it lacks the rural and traditional character of the classic scenic routes and has larger towns and cities with modern buildings and commercial and industrial activity featuring fairly prominently.
Glacier Express route via Andermatt and Chur
www.myswissalps.com/gl acierexpress/gettingth ere
If you prefer to stick with your original plan:
Zürich-Interlaken (see above)
Most scenic route: Lötschberg scenic route (not via the Lötschberg-Basistunnel)
You will get the scenic route if you put “via Kandersteg” into the timetable.
Zermatt-Montreux (see above)
Fastest and less scenic route: via Lausanne. Some connections also go through Bern. The sector from Montreux to Lausanne (about 20 minutes) goes along the Lavaux vineyards and Lake Geneva and is lovely. The rest is comparatively flat and uninteresting in comparison with alternatives - in my opinion ;-)
Most scenic route: Golden Pass
Doing the whole route (Montreux-Zweisimmen-Spiez-Interlaken Ost-Luzern-Zürich) would take about 6 hours 16 minutes. However, if you have already done the Luzern-Interlaken sector, you could shorten this by just doing the Golden Pass sector from Montreux to Spiez, then taking a faster route from there to Zürich via Bern.
>> How far in advance should such train tickets be reserved?
Glacier Express – if you are travelling in April – now. This is a very popular service and reservations for this train are compulsory. Other trains on your itinerary don’t need reservations. If you are just doing the sector between Visp and Zermatt, you won’t need a reservation: although travelling along a sector of the Glacier Express route, the timetable will generate an itinerary using a regional train rather than the official Glacier Express train.
>> Also, how convenient is this this travel with trains (including the exchanges of trains along the way) with luggage, etc.?
Very easy in my experience. Most trains have carriages where at least some carriage entrances have floors level with the platform. Look for the wheelchair sign on doors. Most platforms have stair-free access via ramps and/or lifts. (Whenever you see stairs down to a subway, walk past them and there will usually be a ramp going down to the subway further along the platform). There are plenty of options for storing luggage inside carriages, although when it is crowded, I prefer to travel first class to ensure I can store my luggage conveniently. If you can lift your luggage onto overhead racks, that is sometimes an option.
>> Or are the trains which we'll use in any event (zurich -> Interlaken -> Zermatt -> Montreux -> Zurich via Bern) be scenic enough?
I would prefer more scenic options (see above).
>> As for the specific activities/attractions in each place, I'm still working on that, but what I have at the moment … Interlaken: Jungfraujoch/Schilthorn, Meiringen, Lauterbrunnen, Aare Gorge/Gletscherschlucht, Trummelbach falls.
Interlaken – good for shopping and transport connections, but not particularly interesting or scenic in itself. Wengen, Grindelwald or Mürren are in the mountains and have spectacular scenery.
Meiringen – pleasant enough, but not particularly interesting in itself. It provides good access via train to the Aareschlucht, which is very impressive. It also has a great café (Frutal Tea Room) where you can try the local specialty – meringues.
>> BTW, re the Swiss Travel Pass, does the half price pass grant a greater reduction for Jungfraujoch, for example (since the Swiss Travel Pass only grants 25% to my understanding (but couldn't figure out what the half price ticket grants there))? I'm still deliberating between the two...
Half-fare card gives you 50% discount on the fare Interlaken to Jungfraujoch.
Swiss Travel Pass gives you free travel from Interlaken to Wengen or Grindelwald and 25% discount from there to Jungfraujoch.
>> Yes, I'm (unfortunately) aware that April is not the best of times and this is why I switched the route to be in the Jungfrau area first , before the lapse of Easter - to my understanding most places will remain open there until after Easter (and in Zermatt and Montreux, places closing up after Easter is somewhat less of a concern) - do you agree with that?
The main thing in my opinion is whether the mountain transport that you hope to use is operating or not (i.e. cable cars, gondolas and funiculars). Accommodation and restaurant options will be more limited, but they won’t all be closed. The train up to the Jungfraujoch operates all year.
>> Jungfraujoch/Schilthorn (depending on weather) - is there a specific recommended way to get there and back from Interlaken?
There are two options for getting to Jungfraujoch (from Interlaken Ost) and Schilthorn (from Lauterbrunnen). Most people go up one way and down the other. That is a good option if all modes of transport are operating when you are there.
Jungfraujoch (all by train, operates all year)
Option 1 – Interlaken Ost-Grindelwald-Kleine Scheidegg-Jungfraujoch
Option 2 – Interlaken Ost-Lauterbrunnen-Wengen-Kleine Scheidegg-Jungfraujoch
Option 1 – bus or walk from Lauterbrunnen to Stechelberg-cable car to Schilthorn via Gimmelwald, Mürren and Birg.
Option 2 - Cable car Lauterbrunnen to Grütschalp-train to Mürren-cable car to Schilthorn via Birg.
>> Also, is it 'too much' to go to both Jungfraujoch and Schilthorn (especially given we'll be in Zermatt afterwards)?
They are both very different, both in the trip up and the scenery at the top. If you have the time and the money, and the Schilthorn mountain transport is operating, I would do both. I have attached some photos of each. The Schilthorn photos date from August last year. The Jungfraujoch photos were taken in December last year (hence the Christmas tree!)
>> On the last day maybe a trip in Bern on the way back to Zurich Airport.
If you can visit Bern from Zürich at the beginning of your trip, that would leave you with better options for experiencing scenic train trips. Most routes via Bern are not the most scenic, and during commuter travel times they can be very crowded.