Bernina-Express: fully airconditioned train with wide panoramic windows (cannot be opened) running between Tirano and Chur. The journey goes from Tirano (420 metres above Sea level) to the Bernina Pass (2260m), down to Bever in the Engadin Valley (1700m), then up to the Albula Tunnel (1800m) and down to the Schyn Gorge and Chur (580m).
http://www.rhb.ch/The-World-Heritage.1070.0.html?&L=4
http://www.rhb.ch/Video-Bernina-eng.782.0.html?&L=4
http://www.rhb.ch/UNESCO-Clip-eng.1061.0.html?&L=4
Beside these touristic trains, there are normal trains along the same itineray between Tirano and St. Moritz and between St. Moritz and Chur.
Timetables:
Milano - Tirano:
Milan dp every 2 hrs from 6.20 until 18.20, plus 19.20
Tirano ar every 2 hrs from 8.50 until 20.50, plus 21.50, Fare: 11 EUR
Tirano - St. Moritz:
Tirano dp about every hour from 6.40 until 17.40
St. Moritz ar about every hour from 9.12 until 20.12
exact timetable: http://www.fahrplanfelder.ch/fileadmin/fap_pdf_fields/2012/950.pdf
St. Moritz - Chur - Zurich:
St. Moritz dp every hour from 7.02 until 20.02
Zurich ar every hour from 10.23 until 20.23, then 21.48, 22.48, 23.48
Shortcut Pontresina – Samedan:
If you are in a hurry, you may gain 1 hr if you change at Pontresina AND Samedan instead of St. Moritz.
Example: Tirano dp 9.40 – Zurich ar 15.23
However, it is highly recommended to travel via St. Moritz and to stop over there for a few hours or - better - a few days.