Hello Ckeevoissunn97
Any ticket that is valid for the Tirano to Chur route is valid on the Bernina Express. This train is used by the locals, too.
In Switzerland, we have an integrated ticketing system. It does not matter from which vendor you purchase the ticket, nor does the operating company matter.
The only difference between the Bernina Express and other trains serving the same route: certain coaches on the Bernina Express require a seat reservation.
Kind regards,
Roland