Bernina Express

We took the train from Varenna to Tirano and the Bernina Express from there to St Moritz. This was in 2014, so schedules are likely to have changed, but perhaps will give you an idea of what is doable as a day trip.

We took a morning train (approx 9:20 or 9:30) from Varenna and arriving at Tirano at 10:55am.

We allowed time for lunch in Tirano, a lovely little town where we visited San Martino in Tirano, wandered through the XV century mediaeval Porta Bormina town gate, out into the vineyards, admiring the lived-in homes built into the town wall and had lunch outdoors at a cafe in the small town square. I didn't take note of the restaurant name. The area of the town that we saw was like a step back in time.

That was before we boarded the Bernina Express around 2pm (give or take.)
- arrived at Poschiavo station at 3:02pm
- arrived at Ospizio Bernina station at 3:55pm
- arrived at St Moritz station at 4:36pm (all times are approx and undoubtedly have changed)

You'd have to check return schedules, and perhaps a tighter connection to the Bernina Express, to see how far you could go and still make it back to Varenna. We didn't want to take any chances missing the onward journey to St Moritz.

If you take the regional trains on the exact same tracks, you will have more options in your schedule. The regional trains don't have the panoramic windows, but are much less crowded, better social distancing, no reserve seats, and the windows open for fresh air and better pix. We did that last Sept, 2021, and our regional train barely had anyone else in our car. We passed the panoramic trains, and saw folks shoulder to shoulder, no empty seats in sight. We went from Pontresina to Campocologno and back, and the scenery was so spectacular that seeing it twice was still very exciting. The light changes, the sun sits at different angles, it was all fantastic.
We were overwhelmed with the beauty on this route !

YouTube is full of train videos for this route so you can see destinations

Yes, as Pat said, do not use the named Bernina Express, but the hourly regional trains that do not charge a reservation fee. Alp Gruem will have the most spectacular views, weather permitting. Not sure about lunch options at Alp Gruem. Check when you get a little closer to the time of your trip.

There is only 8 minutes from your arrival in Tirano from Varenna, to make the next train into Switzerland, so hustle to cross the piazza to the RhB station where the Swiss trains depart from. Otherwise you will have a 1 hour stopover in Tirano. Brunch stop?

I'm planning on doing the same...I already purchased a first class ticket on the Bernina Express through the rbb site. I don't remember how much it cost me but it's for a seat in the first car leaving Tirano at 10:06 and arriving in St. Moritz at 12:35.....I leave St. Moritz at 16:14, arriving back in Tirano at 18:39 and plan to take the train back to Varenna.

I'm told that I need to purchase a train ticket....do I purchase the ticket on the RBB site or the Trenitalia site.....do I have to buy a ticket on the same train (972 going to St. Moritz) and (975 leaving St. Moritz) that I have the seat on?

Why do I find this so complicated?! Sandy

It is complicated because that is the way it is.
Many people use a Swiss Travel pass or Half Fare Card or other. To save money, you might look at buying a 1st class Saver Day Pass for your day of travel at www.sbb.ch/en. Unlimited travel on Swiss trains for the day. You already have seat reservations for that day.

you're also dealing with multiple train companies and countries, so yes it can be complicated. perhaps the Man in Seat 61 can help you out?