For transit fetishists like myself, this means the region is leaping a hurdle between our slow, impractical bus system and... the Seattle Subway, a popular vision of an integrated rail network connecting all of Seattle's body parts to each other.
To be square with you: Commissioning this study won't predispose the findings to support a rail system. Planners could conclude that we actually need more buses along this corridor (unlikely) or that we need to extend our streetcar system (possibly). But what I really hope it finds—which evidence from other big cities shows works, regardless of what politicians say about super special Seattle—is that it's time to invest more money into extending the light-rail system. That could be done through a Sound Transit 3 ballot measure, which has yet to be put on the ballot (but voters in Seattle would surely approve in droves, just like the previous two light-rail packages). As Sound Transit explains over here, "By state law, Sound Transit high-capacity transit services must operate principally in exclusive rights of way," meaning that it wouldn't share a lane with vehicles.
The mayor's office says that building a rail line along this route would accommodate up to 26,000 riders per day, a net gain of 12,500 people who currently don't take transit along that route. Weirdly, though, the mayor's office couldn't confirm details of how they got that figure or whether that figure applies to a streetcar or dedicated light-rail line. No matter. That's what's so great about having Sound Transit (and not the city) in charge. They can figure this stuff out, explain it, and build it.