I think, once all three pages are full, new effects randomly replace existing effects. This is how it used to work before they added the extra slots.
Assuming 18 effect slots, and a true RNG, this means they'll have a 1/18 chance of any feature being removed for each new effect.
Without doing the math, I'd wager this value is in the hundreds features added before all negative features are removed. But you could get very lucky and have five removed quickly.
Japan is smaller than California, with several times the population density.
Reframe your thoughts as: taxpayers per mile of track. Then begin to understand.