Scenic train tour - Northern Italy/Switzerland

There's an outfit called Railbookers which gets a lot of publicity, might not hurt to chat with them.

IIRC, the Man in Seat 61 is a fan and you might get a discount if you go through him.

The rail route between Milan and Tirano goes by Lago di Como with a station at Lecco. Unless you want to visit Milan, you can shorten your trip by taking the train from the lake region to Tirano.

The Bernina Express sometimes is sold out, so I suggest buying your tickets in advance. The local train, operated by Rhaetian Railway, runs on the exact same tracks on the same route. Depending on how far you want to travel on the route, you may have to change trains, but they are scheduled so that if you get off one train, there is only a brief wait for the next train. The advantage of the Bernina Express is its bigger windows and commentary on the route. The advantages of Rhaetian Railway are much cheaper prices, almost empty cars so you can move from one side of the train to the other as the scenery shifts, no need to pre-book, and the flexibility of getting off the train in one of the small towns to take a break for lunch, a brief walk, etc.