The Washington Park & Zoo Railway is a 2 ft 6 in (762 mm) narrow gauge recreational railroad with rolling stock built to 5/8 scale. The park's Zooliner, built in 1958, is a replica of General Motors' Aerotrain. The railroad opened in three stages in 1958, 1959 and 1960, and provided transportation between the Oregon Zoo, Hoyt Arboretum, International Rose Test Garden, and the World Forestry Center. The extended line was about 2 miles (3.2 km) long. The service is currently operating on a 1-mile (1.6 km) loop within the zoo grounds. The railway carries about 350,000 passengers per year.
The railroad is operational year-round except in January and part of February, when it is closed for required maintenance. Special events occur during the winter holidays. As of 2018, the price of a ticket was $4.00 for the short 6 minute internal loop, but zoo admission was also required.
Zoo officials announced in September 2013 that the line would be closed for about one year for construction, with the last day of service scheduled to be September 22. The temporary closure was necessitated by the construction of the zoo's new Elephant Lands exhibit, which will also include remodeling of other parts of the zoo grounds. The short-loop route through the southwest part of the zoo grounds was scheduled to be removed permanently, and during the one-year suspension of service a new section of track was to be laid to create a replacement for the short loop. Train operations returned running on the new Zoo Loop line only as of November 22, 2014.
In May of 2018 an online petition was created in opposition to the Washington Park Master Plan's endorsement to remove the "long route", which runs from the Oregon Zoo to the International Test Rose Garden. The master plan primarily called for the removal of all tracks so that it could be replaced by a paved twelve foot (12') wide path. As of September 2018, over 16,000 people have signed the online petition in support of keeping and repairing the, "long route".