Carrying five bus lines and five light-rail lines, the Steel Bridge crossing the Willamette River in downtown Portland has significant public transit clout. But it also carries with it some concerns.

TriMet, the regional transit authority for the Portland metro area, officials recently broached the subject of finding solutions to the overcrowding found on the historic span that doesn’t meet desired seismic code. Adding a wrinkle to the discussion, though, is that TriMet doesn’t own the bridge. Union Pacific does. TriMet simply subleases space from the Oregon Dept. of Transportation to operate on the double-decker crossing.

But the coming costs of repairing aging tracks on the Steel Bridge have forced TriMet to consider all its options. Currently the 106-year-old bridge runs between 10 to 30 trains on the lower span. The upper deck carries light-rail trains, along with buses and vehicle traffic.

The 611 MAX trains that run on the bridge trackway during weekdays has placed a strain on the system, making repairs a coming necessity. TriMet plans to continue to run across the bridge, but wants to remain proactive in finding a future solution.

As the discussion gets going, early ideas have floated up that include upgrading the Steel Bridge, building a new crossing, simply moving MAX light rail lines to other locations or even investigating the possibility of a tunnel under the Willamette River. TriMet officials say that while they don’t own the bridge, as a key user they want to push any effort to find a solution to the current situation.

Looking into the future of the Steel Bridge isn’t entirely new. The Metro’s draft Regional Transportation Plan already lists the Steel Bridge “transit bottleneck,” which impacts the east side’s Rose Quarter near both the Moda Center and Portland Convention Center, as a future need. While the regional plan calls for $700 million sometime a decade from now, the latest word from TriMet projects they will likely need at least $1 billion. Without firm timing, expect TriMet to move quicker than a decade on finding a solution. Already, TriMet hopes to float a bond in the next few years that includes an extension of light rail to Southwest Portland and Tigard that could run near $3 billion. A Steel Bridge solution could potentially join the effort.

Union Pacific said in a statement it has no plans to replace the bridge.

Follow Tim Newcomb on Twitter at @tdnewcomb