Four locomotives (numbered E31 to E34) were built at Seibu's Tokorozawa factory between 1986 and 1987 to replace ageing first-generation electric locomotives on freight services and track maintenance train duties. The DT-20A bogies were reused from former JNR 80 series EMUs, and the traction motors were modified from former EMU motors.
^"大井川鉄道が西武E31形電機を導入！" [Ōigawa Railway introduces Seibu Class E31 Electric Locomotives]. Japan Railfan Magazine (in Japanese) (Japan: Kōyūsha) 50 (596): p.143. December 2010.CS1 maint: Extra text (link)