As to the type of car used for passenger service, the second letter of katakana 'na (ナ)' stands for 'nakagata,' meaning a middle size, whereas 'o (オ)' stands for 'ogata,' meaning a large size.
営業用車のカタカナの2文字目は「ナ」が中型（なかがた）、「オ」が大型の略称である。