The site of Tokyo line is an area in Jet Set Radio Future that can only be accessed once in the game, during Chapter 8. It is a loop of two train tracks that you can grind on. When you access the area, you fight a giant train-like machine that houses the hostage DJ Professor K.
This area is notable because, after your one and only encounter on that level, The Site of Tokyo Line is completely inaccessible. You cannot go there before or after you are absolutely required to, and the area never shows up in your world map, despite an area set aside for it. A possible explanation for the inaccessibility of the area is, after your battle with the train, it grinds to a halt and explosions can be seen all over the train. Presumably that has not been cleaned up, and the area has been blocked off to everyone.
Given The Site of Tokyo Line is a one-time access area, there is no graffiti to tag, nor are there any Graffiti Souls or a Mystery Tape.