It's all very strange, and the story has been changing hourly through Sunday.
The Centovalli site says "8 March 2020 In application of the presidential decree issued today (08.03.2020), the international railway traffic our line is interrupted until further notice.". So how come the (Italian) Presidential decree affects the Centovalli, but not the four SBB/BLS cross-border lines? And what about the RhB (Bernina route)?
According to this evening's Swiss TV news, and confirmed on other news websites, the Italians will be allowing people to cross the border to work, but not otherwise. It appears people in the "lockdown" will be allowed to go to work, to the shops and travel for "medical or other emergencies", but nothing else.