Hi Addy,
>>> Is there usually enough seating in the regional trains to be able to enjoy the ride since it’s a long-ish one?
In my experience, yes, plenty of room – in fact usually half empty. I usually travel in that area in September-October though, and last time was pre-COVID, so the trains might be a bit busier these days. However, they are frequent and have plenty of capacity, so I don’t think you’d have too much to worry about.
>>> Any tips on which regional trains exactly to look out for on SBB to make sure I take the same route as BE?
Between St Moritz/Pontresina and Tirano, there is only one train route, the Bernina line. If you look at the intermediate stations on the route in the SBB timetable, you will see Bernina Diavolezza as one of them, for example, as well as Alp Grüm, so you know that is the right route.
Between Pontresina/St Moritz and Chur, plan the route using Filisur as a via station to get the Glacier Express/Bernina Express route.
>>> What are your thoughts on staying in Pontresina instead of St Moritz? Is it well connected by trains/ public transport?
Yes, Pontresina would be a good choice. It is a station on the Bernina line. Most of the hotels in Pontresina are not near the station, but there are buses to the station if it’s a bit far or not convenient to walk. In your case, I think it would work best if you could stay near the station though, for example at the Hotel Station if it suits your budget and they have a room available:
https://booking.engadin.ch/en/hotels/hotel-station
I haven’t stayed there, only eaten there, but it gets good reviews.
Otherwise, there’s a list here (filtered for those that offer the guest card for free transport and mountain railways)
https://booking.engadin.ch/en/accommodations/hotels?accommodation_type=hotels&later=on&places%5B0%5D=10&facility%5B%5D=railways-included&facility%5B%5D=transport-summer
>>> Can I base myself there for 3 nights & then head back directly into Zurich Airport for my flight or would you suggest stopping over at Chur/ some place else on way back?
Pontresina to Zürich Airport takes 3 hours 52 min, with 3 transfers. Personally I would prefer to stay within an hour of the airport – but I always have a long-haul flight ahead of me, which I couldn’t afford to miss.
In my opinion, Chur has little to offer. It has a lovely old town, which you can see in about 1-2 hours, but that’s about it. I would prefer Zürich, both for proximity to the airport and more variety for sightseeing.
>>> Is there a loop I could do to avoid riding the same way back or I shouldn’t bother with even if I am repeating routes?
If you take my suggestion of going to St Moritz/Pontresina via the Prättigau and Inn Valley on the way there (the aqua-coloured line on the map below), and going back via the Glacier Express/Bernina Express route to Chur (the orange line on the map below), those are two different routes. You can see a map of the Rhätische Bahn network here:
https://www.rhb.ch/fileadmin/user_upload/redaktion/Home_Marketing/Service___Souvenirs/Streckennetz/231109_rhb_linienplan_2023-24_quer_A4.pdf
>>> Just to get an idea, what would your order of preference be between BE/STMortiz, Sils Maria, Soglio and Guarda ?
Soglio and Guarda are the most spectacular for scenery and traditional buildings, as they are both on the side of a mountain, but Sils Maria is also very pretty.
If you had a full day, I would go to Soglio and Sils Maria. Sils Maria is on the way to Soglio, so I would go to Soglio first, and get off at Sils Maria on the way back. I wrote a trip report about Soglio some years ago, which might interest you:
https://www.myswissalps.com/community/thread/trip-report-bregaglia-valley-soglio/
Guarda is in the other direction. I have written a trip report about Guarda as well, which you can see here if interested:
https://www.myswissalps.com/community/thread/trip-report-lower-engadine-guarda-october-2016/
Hope this answers your questions!
Alpenrose