From an article in the April, 1967 issue of the original Seattle Magazine (“Just This, or Rapid Transit, Too?”):

[Seattle Mayor] Braman makes it no secret that he wants to be remembered as the mayor who brought rapid transit to Seattle… His attempts to arouse public interest in the project date back to the spring of 1965…

If you want to get some real context, HistoryLink has several interesting articles about the history of transportation in Seattle. (You’ll find more if you use broader search terms than I did.) For a quick-and-dirty overview, they also have a nice, high-level timeline.