It's a chicken and the egg problem. If the population is denser, than public transport systems like light rail, become cheaper and more effective. If the amount

It's a chicken and the egg problem. If the population is denser, than public transport systems like light rail, become cheaper and more effective. If the amount of people within 1km (i.e. walkable distance) of a light rail system doubles, then people are closer to a train station and it's potential customer base has increased.