This report has been updated multiple times to include information from this morning's briefing about light rail on the 520 bridge.

A report commissioned by Mayor Mike McGinn from consultant Nelson/Nygaard concludes that while it's possible to build light rail on 520, the region's transportation plans will have to be dramatically rewritten to include rail on 520. If the state moves forward with its current "preferred" six-lane option, the bridge would have to be retrofitted in the future and expanded ten feet to accommodate rail—a retrofit that would be prohibitively expensive and environmentally damaging.

Conversely, if the city, Sound Transit, and the state moved forward with planning for light rail on the bridge, it would take about five years to complete planning to add rail to the bridge, a consultant said this morning. The target date to open the new bridge would move back too, perhaps six months to a year, a consultant said this morning.

Nelson/Nygaard consultant Tim Payne and Mayor Mike McGinn present the results of Nelson/Nygaard's study of rail on 520 this morning.

"We have one chance to get this corridor right," McGinn said at a briefing on the report this morning. "The corridor as currently designed virtually precludes light rail in the future."

"If the region moves forward to formally consider plans for light rail on SR 520 today, light rail could be a reality in the corridor," the executive summary of the report says. "On the other hand, if current plans for SR 520 remain unaltered, there are significant, perhaps insurmountable, obstacles to building light rail in the corridor, even if formal planning efforts identify light rail as the preferred option."

Asked to address the fact that the state legislature, governor, and city council seem unlikely, at this late date, to abandon their preferred alternative (the public comment period on the bridge plan closes April 15), McGinn pointed to polls showing that Seattle and Eastside residents support rail on 520, and noted that neighborhood groups and elected officials in the 43rd legislative district, where the Seattle side of the bridge is located, also support putting rail on the bridge. "Five years, 10 years, 20 years from now, are we going to say, 'Boy, we should have built light rail'? It's very, very difficult, if not politically impossible, to to ever build light rail in the future" if the state moves forward with its preferred design, McGinn said. "What I'm asking is that other elected officials in the region jump on the bandwagon that the public is already driving."

McGinn also mentioned the possibility that neighborhood groups might sue to stop the bridge as currently planned, but said he didn't have details about any lawsuits currently in the works. "We do know from the history of highway-building in Seattle that people who don’t like something oftentimes address it in court," he said.

The current preferred design, which includes four general-purpose lanes and two HOV lanes, would have to change in several ways to accommodate light rail, the report says. First, the bridge would have to include enough space between the eastbound and westbound lanes to accommodate rail tracks, which would replace the HOV lanes. Second, the bridge would have to be expanded up to ten feet to accommodate rail on the bridge deck. Finally, the pontoons that support the floating bridge would have to be substantially larger to support the extra weight of rail.

The report identifies five possible corridors that could be studied for future light-rail routes (sorry for the crappy iPhone photo; the presentation doesn't appear to be available online), including a route connecting Ballard to the University of Washington and a route connecting Aurora Avenue in Haller Lake to the UW. The report identifies four options for crossing the Montlake Cut to reach the UW: A high-level bridge, a low-level bridge, a tunnel, or a new bridge parallel to the existing Montlake bridge.