The theory I have now adopted comes from my recent trip from Minneapolis to Portland accessing the airport at both ends via LRT, and then riding the Portland streetcar almost full circle. Rail transit forms an urban superstructure. Guideway transit, esp. LRT makes the city more like a single structure, and makes everything seem closer. The LRT vehicle is continuously running, and if activities are along the path of the vehicle, everything seems quite coordinated. In a way by organizing activities linearly (or multi-linearly), it simplifies the city. Hopping on a train is much like getting on an elevator.
This feels right to me, rail transit is part of the city system. Roads, when empty, are simply paved empty space. They don’t contribute to the functioning of the system, except that they allow transportation to happen.