Updated Thursday, Aug. 9, 2018: Story has been updated to reflect that one of the options TriMet is discussing is moving some MAX lines to other existing bridges. All of the options, however, are long-term plans still in the discussion and research phase.

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TriMet doesn't own the Steel Bridge, but by the sounds of it, the transit agency is leery of continuing to rent its space on the more than 100-year-old span.

At its monthly board meeting Wednesday, the Steel Bridge's future bubbled to the surface during a discussion about an ongoing $20 million construction project to address aging track work on that bridge and elsewhere on the light rail system.

The Steel Bridge is a critical part of the tri-county agency's system. The Union Pacific-owned bridge carries five bus lines as well as five light-rail lines. TriMet subleases space from Oregon the Department of Transportation to operate on the historic bridge.

It's also a bottleneck for trains, buses, cars, and even pedestrians and cyclists on the busy lower span.

Linda Simmons, a TriMet board member, asked whether the agency was ready to "approach an alternative" to the Steel Bridge.

Early concepts are already being analyzed, according to TriMet General Manager Doug Kelsey.

Options could include building a new bridge, upgrading the Steel Bridge, building a tunnel underneath the Willamette River, or moving some MAX lines to other existing bridges.

Kelsey estimated whatever it eventually is, the project would be "at minimum, a $1 billion expenditure."

Kelsey said TriMet, despite not owning the bridge, must champion the effort to bring the issue to the forefront regionally.

Steve Witter, TriMet's head of capital projects, said the agency wants the project to be included in a still-to-be-determined 2020 Metro-led bond measure.

The proposed light-rail expansion to Southwest Portland and Tigard, projected to cost anywhere from $2.6 billion to $2.9 billion, is also likely to be on a big-ticket wish list that could total in the tens of billions.

The Steel Bridge is already on Metro's draft Regional Transportation Plan, a sign of its importance. The placeholder project descriptor puts the estimated cost of "addressing the transit bottleneck at the Steel Bridge and Rose Quarter" in 2016 figures at $700 million. The project timetable is listed as 2028 to 2040.

Justin Jacobs, a Union Pacific spokesman, said he wasn't aware of any discussions about replacing the bridge.

The railroad runs between 10 and 30 trains per day over the span, not including MAX or Amtrak passenger trains.

He said the bridge and others are routinely inspected at least twice a year.

In other TriMet news:

The agency said 1,500 metro residents have qualified for the agency's new low-income fare program during the first month of the state-backed initiative.

"We think this is terrific," Kelsey said at the board meeting. TriMet estimates the program could cost up $12 million to implement and would be able to accommodate up to 45,000 eligible participants.

The low-income fare program was included in the $5.3 billion statewide transportation package approved by the Legislature in 2017.

Thera are many ways to qualify.

Residents need to be 18 to 64 years old and can earn up to 200 percent of the federal poverty level, or roughly $49,200 for a family of four.

Oregon Health Plan, Temporary Assistance for Needy Families and some other federal or state assistance program recipients automatically qualify.

Pass holders will pay half of the regular fare, or $2.50 for a day pass. The low-income pass works as an honored citizen pass does for seniors and people with disabilities.

John Gardner, TriMet's director of diversity and transit equity, said TriMet plans to expand its screening stations and advertising campaign through the end of the year. Right now, people can sign up at 10 locations.

Gardner said TriMet believes it can get up to 15,000 people to sign up in the first year.

"One thousand a month is a really good pace," he said.

Here's more information about the program.