Why light rail? The answer is simply that, when you're building a rapid transit network across an already built-up city, light rail offers you far more flexibility than heavy rail does to move through heavily populated areas. Light rail includes a range of different possibilities in itself. You can have a surface-running streetcar model of light rail, that many people will be familiar with from Melbourne, right through to the light metro option, which you can see in so many cities around the world like Paris, London, New York, and others. So there are a number of different options. Light metro tends to have a completely segregated corridor so that it's not in the street competing with pedestrians and other traffic, and that enables it to go faster. But that segregation then entails a higher level of engineering, which could be going under, being elevated, or in some kind of separated or fenced corridor down the middle of the road, for example.