When the railway was being planned, only the Swiss had significant experience in building rack locomotives, so it was they who won the contract to build the engines for the line. In comparison with some Swiss railways the line is not very steep, and this is reflected in the design of the engines, which are all classified 0-4-2T. The boilers of the locomotives are set at an angle of 9°, to keep the water level over the tubes when the locomotive is ascending the mountain.