Increasing electrification of the Swiss railroad network pointed up a need for electric switch engines. Compared to steam locomotives electric switch engines were ready to run without a lot of preparation time and only required power when they were actually in operation.

After two prototypes (Ee 3/4) were taken into operation in 1923, the SBB purchased a first series of 16 units starting in 1928 from SLM and BBC. These three-axle units could be produced without a pilot wheel set due to technical progress in lowering the weight of their components.

They had a cab at one end of the locomotive, which together with the rather extended roof quickly acquired the more or less affectionate nicknames "Flat Iron" or "Casual Shoe".

A characteristic feature was its Winterthur diagonal drive with a jackshaft driving powered by the traction motor and connected to the driving wheels by side rods. High levels of tractive effort could be realized by the wheel sets connected together without individual axles slipping. This concept was also used at that time on faster locomotives, but its real strength was in those situations where it was a matter of high levels of tractive effort. These locomotives were controlled by a flat sliding step switch with thirteen speed levels that controlled the voltage at the traction motor.

These quaint switch engines were gradually retired starting in 1980. At the end of 1997, the last five units of the existing Ee 3/3 of this series still on the SBB were taken out of service. The "Casual Shoes" sold previously to private operators held on several more years until around 1995/96. Several units remained preserved such as road number Ee 3/3 16318 at Locorama in Romanshorn and road number Ee 3/3 16311 at Swisstrain.

Sources: le-rail.ch