Many visitors will take the Bernina Express day trip from Milan to St. Moritz in one very long day. It’s on the order of 13 hours or so. It involves taking a bus to Tirano and then boarding the train there. You can do this on your own or take an organized tour. This would be very different from just a visit to Lake Como, which can also be done in a single day. To do both would not be feasible in a day. We opted to see Lake Como in the one day that we had free, but if we had an additional day we would have done the Bernina Express trip to St. Moritz and back.