This is the backwards part - you don't get a viable rail option without an existing network of functional, well-patronized buses.
The cases of the different 'Obama streetcars' tell the tale fairly well. The successful ones were built on the backs of a functioning, (relatively) well patronized bus network - looking at Seattles SL*T before the Link, KC Streetcar, or assorted ones. The rail is additive. On the opposite end, you have things like Cincy, OKC, and in some ways Dallas, where the rail isn't additive to the network.
Even the early 90s light-rail mania warns that you can't just plop down light-rail and expect that systemwide ridership (one of the primary indicators of a good public investment) will grow.