berina express

It is a long day on rails - something like 5 hours going up and 5 hours going down, so leaving very early is essential if you want to complete the trip on a single day.

A possibile variation of the tour is going back by post bus from St. Moritz to Chiavenna in Italy, then a short connecting railway to Colico, then train to Milan.

Yes, its possible, you would have to double back along the same route back from St Moritz to Tirano and then to Milan.
The Bernina Railway has one train per hour in each direction (like all Swiss railways), the Milan-Tirano line is less frequent. This would be a very long day, and I am not sure I would recommend it. If you want a fix of mountains, there are others nearer Milan.

The official railway company website is here: https://www.rhb.ch/en/panoramic-trains/bernina-express

You can look up times on the SBB (Swiss Railways) website: https://www.sbb.ch/en/
I looked up some times for a random date in August:

Outbound: 08:20 depart Milano Centrale, 10:52 arrive Tirano
11:00 depart Tirano, 13:11 arrive St Moritz
Return: 14:48 depart St Moritz, 17:00 arrive Tirano
17:08 depart Tirano, 19:40 arrive Milano Centrale

Neither of the above are the actual "Bernina Express" tourist train, just normal regional trains.

BTW, it's the Bernina railway and the Bernina Express. Mis-spelling will mean internet searches do not work.