We should be able to build elevated rail lines on top of roads and, ideally over some structures. The entire point of elevated rail lines is that they can be built on top of existing road infrastructure, so that grade-separated rapid transit service can be extended to a neighborhood WITHOUT having to demolish half the neighborhood for the right of way. Downside is that no-one really wants to live adjascent to elevated rail lines, although noise pollution drops off very sharply. For gameplay purposes, elevated rail would fill a niche between cheap buses (and non-grade-separated trams) and very expensive underground subways, just as it does in real life.