There are five places in Cajon route that throws you to a undesirable yard (or track), though the most famous is at Hesperia (eastbound traffic) diverging from Track 2 into a dead-end spoor (that one of fuel pump). If you are in a freroam you can change any junction of game end-to-end to run the path you want, since you do not make another train change the junction's position back to the default.
In a standart scenario the train (AI or Player) will follow (not always, unfortunately) the path you are programming for it, instead of the default state of junctions. In your case if you wanna make the Player diverge to siding10 (I'm saying by memory, I guess this is the siding's name into which you are diverged by default, if not is siding5) you have to add an instruction for it (destination or go via).
Edited later: Reading again your post, now I'm sure you are talking about the flyout diverging you to the left from track 1 to track 2 at the signal at Natura Crossover before you go to the bridge (in-game is named Frost High Track and Frost Low Track). In this case you have to add a destination (or go via) instruction to Frost High Track, this will keep you going straight (forward), instead diverging you to the left.
Beware that after the bridge (this is why is named crossover tracks) Track 1 turns into Track 2 (westbound run) and vice-versa (eastbound run), and under the bridge happens the same ie Track 2 into Track 1 (westbound) and vice-versa (eastbound).
Who doesn't have dog, hunts with cat.