The problem isn't cramming people into cities, it's cramming people into cities WITH TRAFFIC.
Also, to a lesser extent, with traffic AND NO TREES.
Plant more trees. Plant trees between major roadways and homes. Have fewer cars on the road and more trains.
1 train with 4 cars can move 1000 people. That takes about 15 buses, but over 600 cars (on average, based on typical occupancy; in the best case, you might be able to reduce that to 200-ish). And those cars will require an enormous amount of parking--asphalt that just sits there and heats up in the sun, doing nothing productive.
If you want to move a lot of people around, cars are the worst way to do it, but that's the way that currently dominates. Our cities can be cleaner and less bad for our health if we treat them as places where people live as opposed to places where people DRIVE. City planning for the health of your population actually requires attention to that detail, it's hard to just stumble into it by accident. Plant trees, establish parks, get rid of parking lots, increase public transit into the densest areas. It's all doable, if we want to.