Seeing this again on the new Swiss route has led me to asking this, why do some trains have rear view mirrors?
With passenger trains I guess it's for the driver to check that people on the platform are standing clear of the train before departing (which is the guard's job but no harm in having a second pair of eyes looking from the other end of the train).
It’s to see if any boy racer train drivers are trying to overtake on those winding countryside tracks.