Bernina express train

If you want to explore St Moritz, then why not sleep there instead of Tirano?

I tend to agree with the previous reply. You might find it easier to spend a night in St. Mortiz rather than trying to do it all from Milan as a day trip. It would also be prudent to do a bit of research on sights in St. Moritz. It's quite a posh (expensive) town and I've heard from others that the sightseeing is somewhat limited.

It’s 5.5 hours from Milan to St. Moritz and 2.5 hours from St. Moritz back to Tirano.

Eight hours on trains and buses during one day is an awfully long time.

Note that the train from Milan will not continue past Lecco, Italy. Instead you will need to disembark from the train and transfer to a bus for the remainder of the journey to Tirano. This is due to maintenance being performed on the train tracks.

Having been there. I agree that St. Moritz itself is a rather boring destination outside of winter. Pontresina, just 6 miles away, is more appealing and it’s just a couple miles from Pontresina to the hiking trail to the Morteratsch Glacier.

Another thing to consider is the alternative to the Bernina Express: The local Rhaetian Railway travels on the same tracks as the Bernina Express. It does not offer the huge windows or commentary you'll get on the Bernina Express, but it is much less expensive, has more frequent trains, and is never crowded so you can easily change seats from one side of the train to the other as the scenery changes. Depending on your destination, you may need to change trains but they're scheduled so that if you depart from one train, the next typically arrives within a few minutes. There is no need to pre-book. The "Express" stops for photo ops, so when I used the Rhaetian Railway, I actually reached my destination faster than the "express."