Seeking Help for 1 Week Winter Itinerary

I think you are doing too much doubling back on this stated itinerary, you'll get a sore butt.
Suggest after Zurich, travel to St Moritz, stay 1 night, then take the Bernina Express over and back in one day.

You could take the Bernina Express in the morning. The Bernina Express returns after about 2 hours, time enough for lunch in Tirano and a walk around. Or you could leave later on a regular train and save the reservation fee. If you stay longer in Tirano, the shops will reopen after 2 pm.

Stay another night in St Moritz and then take the Glacier Express to Zermatt, stay 1 or 2 nights in Zermatt, then take a train to Luzern. If you still have not got your fix of scenic railways, you can go by the Loetschberger train from Brig to Spiez, bypassing the Loetschberg Base Tunnel and using the old Loetschberg high tunnel. Then Spiez to Interlaken Ost, then the Luzern-Interlaken Express to Luzern. Now do your 2 nights in Luzern and you can get a 1 hour direct train to Zurich airport in the morning for your flight home.

If your plan is to go all over Switzerland in 8 days, then a Swiss Travel Pass just might be a good value for you. It does not cover the reservation fees on the Bernina Express or Glacier Express, but in winter these are cheaper than summer season.

You are totally doubling back on yourself. You need to consult a map (like this one: https://www.sbb.ch/content/dam/infrastruktur/trafimage/karten/karte-sts-pass-gueltigkeit.pdf )

• 2 days in Zurich (Hotel is booked and cannot change this) - pity, I would skip Zürich altogether
• 2 days in Lucerne, travel from Lucerne from St. Moritz; - This involves going back via Zürich, Chur, Reichenau
• St Moritz to Zermatt Glacier Express - First part to Reichenau is doubling back, then west across the country
• Zermat to Tirano and Tirano to ST Moritz by Bernia Express. - back West to East (which route to Tirano?), now you are back at St Moritz again. BTW, its the BERNINA Express route You now need to get back to Zürich airport, which is a THIRD time St Moritz to Reichenau and a 2nd time from there to Zürich

A better plan:

• Zürich airport - Zürich (as you can't get out of the hotel)
• Zürich - Chur (Bernina Express) - St Moritz - Tirano and back to St Moritz. Unfortunately you need to get back to Chur or St Moritz for the Glacier Express, so some doubling back is needed.
• St Moritz - Zermatt (Glacier Express)
• Zermatt - Interlaken (stay in one of the places further into the mountains south of St Moritz).
• Interlaken - Luzern
• Luzern - Zürich airport.

Chris's plan pretty much mine as well. I suggested Zurich-St Moritz so that you can stay there two nights in the same place and not have to drag luggage going to Tirano and back. The section between Chur and St Moritz, the Albula line, is covered by both the Bernina Express and the Glacier Express, as well as regular Swiss hourly trains.