In the 1950s, the Portland Zoo was getting ready to move to a new, bigger location. The Portland Zoo Railway grew out of a planned kiddy train at the new zoo. When plans were unveiled for the 30-inch gauge line, train fans from around the Portland metropolitan region pitched in with suggestions, fundraising efforts and manual labor. The planning group included members of the Pacific Northwest Chapter of the National Railway Historical Society, the Columbia Gorge Model Railroad Club and the Pacific Northwest Live Steamers.

School children sold "stock" at $1 per share and a children's book called Clickety Clack and the Bandits was sold to help pay for the zoo line. The Portland Zoo Railway began operation June 9, 1958 with the diesel-powered Zooliner, still the primary train used today.

The zoo opened in summer 1959 during festivities surrounding Oregon's centennial celebration. There were soon three passenger trains in the zoo railway's stable. The Zooliner was relocated to North Portland, site of the Oregon Centennial Exposition and International Trade Fair. It was joined by a brand new steam train, pulled by the "Oregon" steam locomotive. A Circus Train was built to operate at the zoo during this time. Parts and equipment from it are still in use as part of the Oregon Express train. Most of the train equipment was built in Portland by local firms at cost or donated. Several parts were also donated.

In 1960 the 1.5-mile extension to Washington Park opened. The line was surveyed by the Southern Pacific Railroad and constructed with the help of the Spokane, Portland and Seattle Railway (now part of the Burlington Northern and Santa Fe Railway) and the Portland Terminal Railroad Co. The Union Pacific hauled ballast donated by school children in Prineville, Ore. at no charge.

In 1978 the railroad was renamed Washington Park and Zoo Railway when the City of Portland turned over operation of the zoo to the Portland metropolitan area regional government. The new name better reflects the service provided by the zoo railway.

Today two of the trains – the Zooliner and the Oregon Express – operate on busy days. Additional rolling stock includes a fire train built in 1959 by Weyerhaeuser Corp., and a work train. They are cared for by both professional staff and volunteers. The Centennial Steam Train only runs during ZooLights and other special event days.

Railway postal cancellation

Until the late 1970s, most mail in the United States was carried on trains. While in transit, mail was canceled using rubber stamps denoting the railroad.

In 1961, the Washington Park and Zoo Railway became one of the first recreation railroads to have its own cancellation stamp. With the demise of the Railway Post Office, the zoo railway is the last operating U.S. railroad with its own authorized railway postal cancellation to continuously offer mail service.

Mail deposited in mail boxes on the zoo grounds or on the locomotives is hand-canceled with the Washington Park and Zoo Railway stamp. It has become a collector's item.