Dual mode means:
1) the can run on electricity (DC current) from overhead wire
2) the have an internal Cummins Diesel to provide power if the overhead wire is torn down (e.g. typically be avalanches)
These engines have been bought as main power for snow fighting, but are also used an normal enines for freight trains -- and YES: the raetian railroad still runs a lot of freight trains on it's narrow gauge network - also on the Bernina line which is the highest rail crossing of the alps (some 6500+ feet a. sea).
Lukas Rosenthaler, Switzerland