Bernina Express from Milan

Cheryl, The Bernina Route (there are more trains running this route than the eponymous "Bernina Express") runs from Tirano to Chur, with St Moritz mid-route. From Milan, you first need to get to Tirano.

You can look up Italian train times on the Trenitalia website ( https://www.trenitalia.com/en.html ). "Milano Centrale" to Tirano takes 2½ hours.
The Bernina route, Tirano to Chur (look it up on the SBB website: https://www.sbb.ch/en ) takes over 4 hours.

So no, you can't get from Milan to Chur via the Bernina route, and back again, all in one day. You need to overnight in Switzerland.
The Bernina route is a route over the Alps between two countries. It is best planned as a route to get from Italy to Switzerland (or vice versa), not as a day trip there and back.

Bernina Express was wonderful for us. We did the opposite direction (Chur-Tirano-Milan-Venice in one day). Some amazing railway architecture (Landwasser Viaduct and Brusio spiral) and unique views over the the Alps.

Our train did not stop or go through St. Moritz. The same company runs another service that goes there, but not the direct Bernina route.

Looking at a random weekday in May, you can leave Milano Centrale at 7:20 am to Tirano and get the Bernina Express at 10:07 am to St Moritz, arrivng at 12:36. The return Bernina Express is at 3:11 pm to Tirano, connecting to a train back to Milano Centrale, arriving at 8:40 pm.

That will give you enough time in St Moritz for a walk around town and lunch. Round trip Milan to Tirano is 23 EUR. you could get a Swiss Saver Day Pass if it is weekday for 52 CHF buying 60 days in advance. Then there is the Bernina Express reservation supplement costing 28 CHF r/t. Worth it? You tell me. That is about $100 US all told.

thank you everybody,,,now something more to think about...