The word otaku has come to represent fans of anime, manga and 2D gaming culture, but the word is a lot more versitile and can be used for any kind of obsession a given person might subscribe to. There are female perfume otakus who travel the world world collecting rare fragrances, military otakus who engage in war games with their friends, "bus otakus" who love riding in decommissioned busses from the Showa Period, female history otakus (rekijo) who blur the line between history and pop culture, and RC otakus like the guy in my neighborhood who's flown RC planes in a nearby field every Sunday...for more than 15 years now. Having so many trains, Japan naturally has its share of densha otaku (railfans), and when I was an English teacher I had students who would tell me about their dream of riding such-and-such train line in some obscure part of Japan someday. A few weeks ago I wrote about Japanese traffic signals, and in the course of googling up some art I discovered there are "traffic signal otaku" in Japan, who delight in taking pictures of bizarre or interesting traffic lights and sharing them with others.
This image is porn to a Japanese traffic signal otaku.