Why do we talk about rapid transit? Because it embraces a range of different modes from heavy rail, light rail, busways that are running on dedicated grade separated tracks, and, essentially, it's high-frequency transport that is not competing directly with traffic. That's the working definition. So rapid transit is the thing that's important, not necessarily the mode, and then the idea is you make decisions about mode depending on the trade-offs between frequency cost and the terrain that you're moving through.