The very detailed track map of the Rhätische Bahn photographed on the information post of the reconstruction of Zernez station

These REX trains provide direct connections from the Prättigau to the Engadin and to St. Moritz (Engadin Regio trains evade St. Moritz and serve Pontresina instead, providing connection from the Albula trains to the Berninabahn while the Northernmost part of the Unterengadin is served by the Scuol-Tarasp to Disentis/Mustér REX trains over Vereina-Klosters-Landquart and Chur). Plans are to have these trains in an hourly regular timetable from 2015 so the change of trains at either Sagliains or Samedan is going to be unneccessary for travelers of the Engadin and the morning S-chanf to St. Moritz commuter push-pulls are going to be replaced by longer REX trains from as far as Landquart too. They of course travel over the triangle on the Engadin end of the Vereina base tunnel (Abzweigung Sasslatsch II). The original EngadinStar was a weekend-only train hauled by a Ge 4/4^II but for convenience I'll call all these in fact unnamed REX trains EngadinStar. They often bring freight cars to Samedan thus taing load away from the heavy freight trains and mixed REX-freight trains on the Albulabahn that tend to have a lot of passenger cars and therefore a limited freight capacity.
The GlacierExpress (GEX) panoramic car is probably returning from a repair from the shop at Landquart.