Although nearly all tracks in Switzerland are electrified, there are still a few short sections in yards or in industrial sidings which don't have a catenary. So trains which operated over these short non-electrified parts either needed a diesel locomotive over their full length or the locomotive had to be changed. To overcome this and to reduce its carbon footprint, the SBB ordered an electro-diesel shunter.
Stadler won the tender with their Eem 923 “Butler” that was based on the Ee 922 electric shunter. 30 have been delivered in 2012 and 2013. The power in electric mode was increased to 1,500 kW and a 336 kW diesel engine was added for the non-electrified sections. While its predecessor can only reach 40 km/h when running under its own power, the new locomotive can reach 120 km/h.
