María Carmen I bought a ticket from St Moritz to Tirano by train.
You did not buy a ticket for a train, but a ticket for the route St. Moritz to Tirano. That ticket entitles you to travel by train on the route, one any train available.
And a nice thing is that you can also interrupt your trip and continue on a a following train, so you can eg. get out at Alp Grun, ad Diavolezza etc, and then continue.