What is this symbol used in Kabaneri of the Iron Fortress? While watching Kabaneri of the Iron Fortress, I saw this symbol during the opening song:



But what exactly does this symbol stand for? Is it ever mentioned what this stands for? Have I missed something? I also think I saw it once in the first episode, painted on the train, so I'm guessing it is the symbol for the train itself, but I'm not sure... I just want to know for what is this symbol used.
According to this post on the official website, this is the 浄化マーク, meaning "purification mark". It is derived from / inspired by older variants of the character 六, which is the "Rok-" in the phrase "Rokkon Shōjō" and means "six". This symbol is written on things to indicate that the things in question are "pure" (free of kabane, presumably). 

I don't recall this being mentioned in the anime.