The Class E31 was a class of four DC Bo-Bo wheel arrangement electric locomotives operated by the private railway operator Seibu Railway in Japan until March 2010 and subsequently by the Oigawa Railway.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. Given a list of categories: company, educational institution, artist, athlete, office holder, mean of transportation, building, natural place, village, animal, plant, album, film or written work, what category does the paragraph belong to?
Mean Of Transportation