Bernina Express

The Lugano to St. Moritz portion of the Bernina Express is by bus and does not run in winter.

Apologies, I meant the train from Tirano to St Moritz, or am I mistaken and that is also a bus which does not operate in winter?

Your train ticket allows stopovers along a prescribed route within one day. But if you want a seat in the panorama car, then you pay for a seat reservation on the train departing Tirano at 9:40 daily. Many stops on this route are by request only, so you'd have to make that request in the train station or with the conductor soon after boarding.

Tirano to St Moritz is a train. This is called the Bernina Line. And as on all Swiss rail lines, there is one train per hour in both directions, 7 days a week, 365 days a year.
The Bernina Express is an extra tourist train on this route with panorama windows.

Lugano and St Moritz is a bus, no rail line. It is summer only, one bus in each direction per day.
There is also a summer-only daily bus from Lugano to Tirano, designed to connect to the Bernina Express extra train.

But if you are coming from Milan, you can get a train direct from Milan to Tirano, and change there for the Bernina Line, no need to detour via Lugano. You can then go to St Moritz, and on to Chur, where you can connect with the rest of the Swiss rail network.

See this map of the Swiss rail network: https://www.sbb.ch/content/dam/sbb/de/pdf/freizeit-ferien/ferien-kurztrips-schweiz/internationale-gaeste/sts-geltungsbereich_en.pdf

I am unclear as to whether we can stop overnight at Pontresina (near Mt. Moritz) and then proceed the next day on the same ticket. It actually says there is no Bernina Express from Pontresina or Mt. Moritz on to Zurich. Can anyone help? This is a trip in October.
Thanks !

Lynn, it is St Moritz, not Mt.

As mentioned by Chris just one post earlier, the Bernina Express starts and finishes at Chur. It does not continue to Zurich. You need to change trains in Chur.

It actually says there is no Bernina Express from Pontresina or Mt. Moritz on to Zurich

What "it"? Where are you looking?

Lynn, If you stop of somewhere overnight, you then need two tickets. One from wherever to Pontresina on day 1, and another from Pontresina to wherever else on day 2.

Don't get hung up on whether the train is called the Bernina Express, that is just one extra tourist train. There is one train per hour on every rail route in Switzerland, including Pontresina to St. Moritz.

Do you actually want to go Pontresina to St. Moritz, or Pontresina to Zürich? If you want to go to Zürich, buy a ticket to Zürich.

And the Bernina line ends at Chur. You always have to change there for another train to Zürich or anywhere else in Switzerland.