I mistakenly deleted this post a couple months ago. I was spooked by at strange email....However, I had copied it and now that my trip is over (and will be revealed soon!) Here are the very good instructions that I followed ! "Note that there are two routes to Pontresina from Luzern, and one is definitely better than the other. See the routes on this rail map:
https://www.sbb.ch/content/dam/infrastruktur/trafimage/karten/karte-sts-pass-gueltigkeit.pdf.sbbdownload.pdf
If you put in a journey from Luzern to Pontresina on the Swiss rail website (www.SBB.ch or www.rail.ch for English), it will show journeys taking 4:17 which go Luzern—Zurich or Thalwil—Landquart—-via Klosters, Zuoz and Bever to Samedan, where you change for the short ride to Pontresina. 3 changes in all. Do NOT go this way.
The other route takes a bit longer, 4:33, but it avoids the long tunnel before Klosters and takes you over the beautiful and famous Landwasser viaduct. This route is Luzern —Thalwil—-Chur...via Thusis, Flisur and the shorter Albulatunnel to Samedan, where you again change trains for the short ride to Pontresina. Same number of changes (3) but a much more scenic journey."
Thank you for sharing this very good tip! Indeed, the Albula railway is much more scenic than the route from Landquart through the Vereina tunnel.
I will add that there is another option if you are happy with a longer and more complicated journey: you can travel from Luzern to Andermatt via Göschenen, and cover most of the Glacier Express route from Andermatt to Samedan (changes in Disentis and Reichenau or Chur). It takes an extra 2 hours (and 2 extra connections, but at very tiny stations), but it basically ticks Glacier Express off your list!