Bernina Express day trip from Varenna

Real easy!

For the actual Bernina Express, which has slightly more comfortable cars and fewer stops (but oddly slower) in return for a surcharge, you depart Varenna at 0824 change at Tirano (0952-1003) and arrive Pontresina at 1224. You can also use other trains from Varenna which connect into ordinary train at Tirano, 0724 gets you to Pontresina at 1052, 0924 at 1252. There are retrun options most hours taking around three and a half hours.

But I would take the 0824 departure, ride the actual Bernina Express and go on a little more to St Mortiz, arrive 1236. Then take the Swiss Post Bus at 1354 to Chiavenna in Italy (via the Maloja pass) arrive 1553. Local italian train at 1644 to Colico arrive 1715, depart 1737 arrive Varenna 1805. That's a day trip worth doing!

One point on StevsB's plan: Take the earlier 7:24 train from Varenna-Esino station. With the 8:24 departure, you only have 11 minutes to change trains, and you actually have to change train stations. Now they are across from each other on a big piazza, but you will have to walk along the platform, perhaps some ways to the station exit. Then eyeball the Rhaetischebahn station and walk to it and walk out to your assigned carriage. If you have not already printed out your ticket and reservation, you'll have to get those. And while Swiss trains run pretty much like clockwork, Italian trains might not be quite so prompt.

So take the early train, and instead of stress, you'll have time to stop at one of the snack-bar/pizzerias next to the Rhaetischebahn station and have a relaxing cup of coffee and a cornetto.

Thanks for the advice!