Bernina Express at St Moritz

Yes, they use the same tracks. and you can board the Bernina Express at any station in any direction if you have a seat reservation. However, the Bernina Express does not call at St Moritz in the north bound direction. You would have to catch it at Pontresina, The Glacier Express northbound does depart St Moritz 3 times in the morning during the summer season and it also uses the same tracks, the Albula Railway. Again, a seat reservation is required.

If you want to see the Landwasser Viaduct, just take one of the frequent regular train. You can connect at Fillisur to a small local train that goes over the Landwasser Viaduct and you get off at Schmitten Landwasser and hike down and view the Viaduct from below.

Rhb (Rhaetische Bahn) has both standard trains over its whole network and the two tourist trains, Bernina Express (Tirano-Pontresina-St. Moritz or Tirano-Pontresina-Chur) and Glacier Express (St.Moritz-Chur-Disentis and then to Brig and Zermatt over the FO railway). Tourist trains have panoramic cars and are much more expensive, but standard trains are as good, cheaper and much more frequent.

A point easily overlooked is that the Bernina railway was once an indipendent line, built to a different technical standard than the rest of the network, namely a different electrical tension. Trains can switch tension in Pontresina (and from there go to Chur), but not in St. Moritz where the two systems are not linked within the station. This means that a Bernina Express joining all four places, Tirano-Pontresina-St.Moritz-Chur, is not technically possible.

It would be possible to ride the Glacier Express between St. Moritz and Chur, I do not know if it would be worth the extra expense. A standard train would be as good.

More information on the Viaduct Shuttle.
https://tickets.rhb.ch/en/pages/viaduct-shuttle

The regular trains from st. Mortiz to Chur all have a special photographers lounge at one end. This compartment has big windows, that can open. So you do not need to take a special panoramic train to enjoy that route.

WengenK that is great information to know. I like those regional trains but I'd never discovered the special car, Another thing to try out! Maybe this summer.