The bridge was completed in 1912 and replaced the Steel Bridge that was built in 1888 as a double-deck swing-span bridge. The 1888 structure was the first railroad bridge across the Willamette River in Portland. Its name originated because steel, instead of wrought iron, was used in its construction, very unusual for the time. When the current Steel Bridge opened, it was simply given its predecessor's name.
The bridge was designed by the engineering firm of Waddell & Harrington, which was based in Kansas City, Missouri, but also had an office in Portland.:7, 52 The structure was built by Union Pacific Railroad and the Oregon-Washington Railroad and Navigation Company (OWR&N) at a cost of $1.7 million (equivalent to $44 million in 2018). It opened in July 1912 to rail traffic and on August 9, 1912, to automobiles.
The 1888 Steel Bridge (upper deck) had been crossed by horse-drawn streetcars from the time of its opening and then by the city's first electric streetcar line starting in November 1889;:23–25 when the present Steel Bridge opened in 1912, the streetcar lines (all electric by then) moved to it, starting on September 8, 1912.:7 Streetcar service across the Steel continued until August 1, 1948, when the last car lines using it, the Alberta and Broadway Lines, were abandoned. A single line of Portland's once-extensive trolley bus system also used the bridge; the Williams Avenue line crossed the Steel Bridge from February 1937 until October 9, 1949.:31 Many years later, in 1986, electric transit vehicles returned to the bridge in the form of MAX Light Rail and later the Portland Vintage Trolley.
In 1950, the Steel Bridge became an important part of a new U.S. 99W highway between Harbor Drive and Interstate Avenue. Harbor Drive was removed in 1974 and replaced with Tom McCall Waterfront Park.
A westbound MAX Blue Line
train crossing the bridge in 2009. Four of the five MAX lines cross the Steel Bridge. More than 600 MAX trips cross the bridge each weekday.
In the mid-1980s, the bridge underwent a $10 million rehabilitation, including construction of the MAX light rail line of TriMet. The span was closed to all traffic for two years, starting in June 1984. It reopened on May 31, 1986. Completion and testing of the light-rail tracks and overhead wires across the bridge took place during the months that followed, and the light rail line opened for service on September 5, 1986.:37–39
A single-lane viaduct that connected the bridge's east approach to another viaduct (still in existence) that takes traffic from southbound Interstate 5 to Interstate 84 was closed in 1988 and removed in 1989, as part of roadway changes intended to improve traffic flow around the Oregon Convention Center. The center was under construction at that time and opened in 1990.
The lower deck of the bridge was threatened by major floods in 1948, 1964, and 1996.
In 2001, a 220-foot-long (67 m) and 8-foot-wide (2.4 m) cantilevered walkway was installed on the southern side of the bridge's lower deck as part of the Eastbank Esplanade construction, raising to three the number of publicly accessible walkways across the bridge, including the two narrow sidewalks on the upper deck. The bridge is owned by Union Pacific with the upper deck leased to Oregon Department of Transportation, and subleased to TriMet, while the City of Portland is responsible for the approaches.
The average daily traffic in 2000 was 23,100 vehicles (including many TriMet buses), 200 MAX trains, 40 freight and Amtrak trains, and 500 bicycles. The construction of the lower-deck walkway connected to the Eastbank Esplanade resulted in a sharp increase in bicycle traffic, with over 2,100 daily bicycle crossings in 2005. MAX traffic across the Steel has tripled since 2000, when only the Gresham–Hillsboro line (now the Blue Line) was using the bridge, to 605 daily crossings (weekdays) as of 2012.:38 This resulted from the addition of three more MAX lines during that period: the Red, Yellow, Green Lines.
In summer 2008, the upper deck was closed for three weeks to allow a junction to be built at the west end connecting the existing MAX tracks with a new MAX line on the Portland Transit Mall. A change made at that time was that the two inner lanes became restricted to MAX trains only, with cars, buses and other motorized traffic permitted only in the two outer lanes.
In 2012, the Steel Bridge celebrated its 100th birthday. The Oregonian called it the "hardest-working" bridge on the Willamette River: "Cars, trucks, freight trains, buses, Amtrak, MAX, pedestrians, bicycles — you carry it all."