When you emerge from the Rhaetian rail station in Tirano look to your slight right, across the small square, and you will see the Trenitalia station.

You can purchase your ticket for Milan in that station. On the way TO Milan you are going to pass right through the station you'll eventually use, or could use, for a trip to Bellagio, and that is Varenna-Esino.

When you say "Como?Bellagio" are to speaking of the CITY of Como or are you talking about the mid-lake region where Bellagio, Menaggio, and Varenna are located?

Day trip:
Milan Central Station to Como.
Then boat to Bellagio.

Then late that evening back to Milan on the train: Varenna to Milan.

I was thinking about riding the boat to Verenna that morning and pre-purchasing ticket for 7:00PM or so, to save time that night.

I copied and pasted your instructions in my notes.
Thanks Dukey1

Why don't you buy the Varenna to Milan ticket when you're at the Milan train station?

Good idea. I will check that out when we arrive in Milan.

When I took the Bernina to Tirano last fall, the ticket office in Tirano was closed. Know that if that's the case, you need to go to the Bar inside the station to buy the ticket.

That little cafe/bar in the Tirano station can get lively and the folks are very friendly. Also be aware that the Varenna-Esino station is unmanned although there are ticket machines.

For that matter, unless things are confusing for people, you probably could buy your furure Varenna-Milan tickets at the Tirano station as well.

Also, those Triano-Varenna-Milan trains are regionals. They do have one small First Class seating section (which I wouldn't spend money on) and the rest of the seats are in Second. Reservations aren't required. The busiest I have ever seen that route is between Centrale and Monza and on into Lecco and vice versa. It can have a lot of people who commute back and forth to work on it between those stations I mentioned but still you shouldn't have any trouble getting places to sit.