Tax policy is key here. The most effective approach is to reduce underdevelopment of prime land, and this can be achieved through shifting taxes away from improvements and onto land value. see: Land Value Tax
Tax reform, zoning reform, and light rail are not mutually exclusive ideas. The density issues we face aren't so much about too much or too little density, but uneven density.
We have urban wasteland regions, and then nearby you might see very crowded areas. Car culture and zoning play a big role in that, but land speculation ought not be underestimated. Every vacant lot or abandoned building is owned by someone. If it is left vacant, that is because the owner is choosing to keep it that way. A high tax on land value, regardless of whether it is in use, would ensure that those who aren't making use of prime locations sell it off or rent it out affordably to people who do intend to make use.
It isn't as if vacant lots provide breathing room, and make cities nice places to be. Parks do that, sure, but not vacant lots. Vacant lots just attract weeds, vermin, and used heroin needles.
I admit, my focus on tax reform is actually because it would raise wages, reduce unemployment, and improve economic efficiency... and thereby improve the rate and trajectory of social progress. But the benefit of more rational land use is a nice little bonus on top of that.
