Staying in Lake Como or Lake Garda - day trip Bernina Express?

Lake Garda would be very inconvenient, maybe impossible, but fairly easy if staying near Varenna on Lake Como.

It’s a bit more than a daytrip for the full route of the Bernina Express from Tirano, Italy to Chur, Switzerland.
The entire route takes more than 4 hours. I rode the entire route a few years back,

From Varenna on Lake Como to Tirano takes about 1.5 hours. It’s then about 2 hours from Tirano to St. Moritz.
In September, we left Lake Como for Tirano and transferred to a train on the Bernina Expess route to Pontresina, just shy of St. Moritz. This is the most spectacular half of the route. We then overnighted in Pontresina before returning to Italy the next morning.

With a pre-set itinerary, splitting the roundtrip over two days lets you relax and means if you hit rainy weather one day ( which is what happened to us) you still have another day with a chance for sunshine, so you can actually see the panoramic views of the Alps.

While staying in Varenna, I took the Bernina train route as far as Pontesina (shortly before St. Moritz), had a late lunch and spent a couple hours there, and then returned to Tirano and then Varenna. From what I had read, Pontresina was more appealing to me than St. Moritz. It was a beautiful ride and fine for a day trip. Because of the way the clouds and sun shifted, the view on the return trip with the sun setting but the sky clearer was a slightly different experience that the ride up though both were impressive. By taking the "regular" Bernina train on the same route, I paid less and had more choice of train times than with the express.

Agree, for taking the Bernina Expreß it is much better staying at Lake Como. And from all of the villages around the lake, Varenna is the best. From there you can explore the whole lake easily by ferry, and the eastern side of the lake by train, including the trip to St. Moritz.
Oh, and to add: March/April is a really beautifull time here. So many flowers in the gardens.

Too long for a day trip, I think, but don't let that stop you. Just add an overnight in Chur, make it a two-day trip, and that would be an amazing adventure.

We did the Bernina Express in June a few years ago, and it was a highlight of our time in Europe.