1900s. Our current 18-mile long Springwater Trail is a former rail line. The Kenton neighborhood in North Portland, built by the Swift Meatpacking Company, was anchored by a streetcar line along Denver Avenue.
The blog Cafe Unknown has a great post on remnants of Portland's streetcar system here and a post about the city's fleeting experiment with electric buses in the 1940s.
Martha Bianco's dissertation, Private profit versus public service : competing demands in urban transportation history and policy, Portland, Oregon, 1872-1970, takes a comprehensive look at Portland's streetcar system and other aspects of urban transporation from a public policy perspective. It is available at PSU's library. She has also published several articles including, “The Decline of Transit—Corporate Conspiracy or Failure of Public Policy?: The Case of Portland, Oregon,” Journal of Policy History, Vol. 9, No. 4, Winter 1997: 450-474.
Belmont Street at 33rd Avenue, circa 1915
All of these images and more can be accessed here.