The book (and all FGG books in general) default to the middle track. However, with any adventure book of this size (like Rappan Athuk as well, for example), you really have the option of using whichever XP track you want depending on how quickly you want it to go. There are so many options for random encounters along with the fact that not every single numbered location must be encountered (and in fact is really unlikely to happen without A LOT of GM nudging) means that there is a high variation in XP dispersal that can occur. So no single track is required. It can be tailored to suit whatever pace of game you'd like to have. In fact, it could change in mid-course if you wanted (say your group doesn't like wilderness so you fast track them through the Desolation to Tsar, but they LOVE ruins exploration so you slow it way down, and then they're okay with dungeon crawls but don't have quite the same affection for them so you switch it to the medium track in the Citadel, for example). Because Tsar is both extremely sandboxy and pretty much encompasses a whole campaign, it is necessarily mutable to whatever the individual GM/campaign needs in that regard.
