The Apocalyptic Japan Track (or AJT so I don’t have to type that again) is a series of three books — soon enough to be four — that split off from the main Timeline Zero track and follow a number of characters through their experiences in Timeline 1287 Left Sub 6.
Timeline Left 1287 itself is known to have at least six sub-lines and possibly some supra-lines. Timeline 1287 Left Sub 6 may have fractured into a number of sub- and supra-lines of its own over a period of a century, between 2050 and 2150, but evidence of this has disappeared with the conjectured probabilities having folded back into Left Sub 6 at the end of that period.
There’s more about this timeline in the wiki. The books in this side-series are as follows (and should be read in this order):
Planned and in progress are
Shade of the Sakura
桜の木陰
On the Other Side of Time
時間の向こう側で