
Head on over to Crosscut.com for an excellent article by amateur local historian Robert Ketcherside, "Why light rail was predestined for MLK Way."
Whether or not you agree with his conclusion (others feel the corridor should have been Rainier Avenue S. or E. Marginal Way S.), it's a great piece and makes wonderful use of Googe Books to highlight elements of the Bogue Plan, which was soundly defeated by Seattle's voters in 1912.
According to Historylink.org, it would have included, among other things, "a giant train station on the south shore of Lake Union, a Civic Center complex of government buildings in the recently leveled Denny Regrade, a rail transit line linking Seattle and Kirkland via a tunnel beneath Lake Washington, and possible acquisition of Mercer Island as a city park."
Ketcherside specifically makes reference to its proposals for arterials and highways, the location of which "calls for much the same study and talent as are needed in the location of a railroad."