Ok you asked
Single red can be the rear of a light loco or loco on the rear of a push pull.
But! It's not mandatory as the did away with the requirement for a rear light in the 90's.
It can also mean wide load if displayed on the front with two whites as in your photo.
Three reds is an emergency warning.
That info courtesy of Gion Caprez driver and historian of the RhB.