1960s movie about opera diva


I am looking for a movie I saw as a child. The main character is an opera diva. Scenes I can remember are a duel with swords vs. a rival diva, and a train trip in winter to Russia for a command performance for the Tsar.

It is in colour.

In the end, she sings in the opera Tosca (I think) and her lover is the tenor. His rival hires an assassin who hides backstage and shoots him for real as the firing squad in the opera fires at him.
Please help me find this movie. Thanks

"Money won is twice as sweet as money earned."

Thanks. But I don't think so. It is not a musical and the story line seems off.