CityLab offers more details about how Seattle succeeded in accommodating a large increase in population with an expanded bus system. A good representative quote:
The first lesson for any transportation planner looking to reverse-engineer Seattle-style success: Make room for buses.
Seattle is one of the few cities to escape the transit death spiral, where budget cuts lead to reduced service hours, which leads to lower ridership. Seattle has created a virtuous circle, where good bus service leads to higher ridership, which supports better service.
The article also mentions that the Nashville, TN transit measure mentioned previously here was defeated in an election last month.