Total Recall and Avatar take place in the same universeGoing with the idea that the story is real. In Avatar Jake Sully had a line "they killed off all the green" when talking about Earth. In Total Recall's backstory there was a war that decimated most of the world and vegetation. Unobtanium is necessary to make the layered floating cities (hence why it's worth sending so many people out to mine and get it).
The reason the "worldwide gas attack" makes no sense is that it is a government lie. It was actually aliens that killed almost everyone.How does a normal gas attack work? Typically, a bomb is detonated, scattering gas over a medium-sized area, like a city, persisting in the area for days at a time, killing everyone there. Eventually, the gas either degrades, or dilutes enough that it is no longer dangerous. How does the supposed "gas attack" work here? Worldwide warfare, eschewing nuclear or biological WMDs, somehow becomes one where gas attacks are widespread. After this is done, it supposedly leaves about 99% of the earth, aside from two areas, uninhabitable forever. Aside from a few tiny settlements around the world, supposedly everywhere else in the world has gas that will kill you instantly if you don't have a mask, and that gas never degrades, and never moves into the other two areas of the world. Sound suspicious? That's because it is. It's true that there was a large-scale war that killed many people in the world before the film started. However, nuclear weapons have fallout, and biological weapons spread by air currents, so what is a weapon of mass destruction that can kill huge swaths of people in the world, but still leave other parts untouched? The only possible answers: Radiation, kinetic kill weapons, and conventional weapons. So, what did kill most of the world? Lasers? Orbiting rods? Satellite drop? Simple machine guns? No, those are all too small scale. Those are things armies use to exterminate other armies, and while they can be used on civilian populations, armies of humans won't typically kill unarmed civilian populations which could be better put to use. Even Hitler used Jews in work camps instead of killing them all. So, before the film starts, there has to be a war that a. Kills huge swaths of people, eliminating civilian centers of production alongside armies b. Spare two sections of the earth the bulk of the destruction. Not only that, two relatively significant parts of the earth, while less significant parts like Indonesia and Mongolia are destroyed c. Have effects that don't bleed out into other areas of the world, like CBRN do. Who would do that? Aliens. Maybe this takes place in the same world as Independence Day, Battle: Los Angeles, or The War of the Worlds, or maybe they are some other aliens, but the world destruction shows wonton extermination such that humans aren't known for in warfare, and almost bureaucratic precision. It looks like an alien fleet started methodically destroying the world, with refugees fleeing out of their paths, making sure to destroy entire areas 100% before moving onto the next one. Refugees flooded to the UK and Australia, and before the aliens could finish their precise dirty work, germs, armed resistance, or some other emergency recalled them before they were finished, leaving those two areas free from destruction. So, why don't the reaming humans be fruitful, multiply, and rebuild the earth? Because the British government doesn't want them to. It tells everyone that the aliens left a gas behind that will kill those that venture back to the destroyed lands. But those points of light in the beginning showing settlements says otherwise. There is actually a thin strip of radiological salted earth around Brittany in Northern France, as well as mines all around Australia, such that if anyone tries to leave, they either die by mines, or die of radiation because they're wearing chemical suits while radiation kills them, similar to the movie K-19 the Widowmaker. It does this because it wants control. Cohaagen has control of almost every citizen in the world, making him a world emperor. If people learned the real truth, that nature was reclaiming the bulk of the world and that scattered, schizo-tech societies were slowly rebuilding outside of his two nexuses of control, they would want to expand, causing him to lose his grip on his power. In fact, Cohaagen's final solution is this: wipe out Australia, where the bulk of Asians congregate, and fill it with Englishmen - loyal, homogenous, bland Englishmen that would never dream of rebellion. He would then take a small army of exclusively robots and, unknown to the general public, will kill the last pinpricks of human settlements outside his control. This will ensure that his dying days are spent in hedonistic pleasure as king of the world, with no hope of rebellion anywhere in the world, no threat to his power, and no worldwide expansion that could cause complications.
The reason humanity hasn't abandoned its dying world for greener pastures in space is due to an Ur-Quan slave shieldTotal Recall is actually an Alternate History of Star Control. There are two changes. First, the Small War of 2015 became a large war. Rather than only a dozen or so nuclear weapons being detonated, instead, World War III broke out. Factions and alliances were formed, such that a billion people died in the first year of the war. The conflict eventually encompassed every nation in the world (even Switzerland!) except for the United Kingdom and Australia, who remained fiercely neutral. The conflict ended around 2025, but left most of the world uninhabited. Second, due to better luck in previous wars of enslavement, the Ur-Quan Kzer-Za are able to get to our sector of space about 30 years earlier, and make contact with Humans before the Chenjesu do. In no position to offer even the slightest resistance, and of no use to the Ur-Quan as thralls, the Humans quickly surrender without any resistance. The Ur-Quan still destroyed the Pyramids though, for reasons no Human can explain. The Ur-Quan erect a slave shied around Earth, preventing humans from ever reaching Earth's orbit again. Therefore, when Total Recall starts, there are no satellites, no space colonies, no moon or Mars colonies, and in fact, if anyone ever tried to lift into space, their craft would disintegrate before they ever exited the Stratosphere.
"The Fall" does not actually fall through the earthAlthough the train is called "The Fall", it is doubtful that it actually falls through the earth. It is simply impossible to fall to the Earth's core and then use the momentum to pass the core and move from Britain to Australia in 15 minutes. The train is more likely to have engines and artificial gravity generator to function. At the beginning, when the "fall" makes everything weightless, the artifical gravity matches direction with "normal" gravity to enable the guards to walk around the corridors. As the train approaches the Earth's core, the artificial gravity is balanced with the train's speed to achieve weightlessness (for pretty much no reason except Rule of Cool). Finally, when the train passes the Earth's core and begin to "resurface", artificial gravity changes direction to the opposite of the Earth's gravity, this time to keep the guard from being glued to the floor.
Outside of the two settlements, the world has turned into Gamma WorldPretty self-explanatory. The gas was mutagenic as well as mostly fatal, meaning there's all manner of weird men, mutans, manamals, and plants outside the city and colony. Total Recall (1990), which, we can then assume, ended with a Fade to White because he was actually lobotomized at the end. They both referred to themselves as "Douglas Quaid" and remember a relation with a woman named "Melina" because those names are just standard elements of the program, implanted into them as false memories.