Casdade Bicycle Club is celebrating the still-under-construction Linden Ave Cycletrack as a Seattle first. But is it really? In my travels I've noticed a few similar facilities, many of which serve a similar purpose (part of a regional trail), that already exist.
Several sections of the Burke-Gilman Trail might reasonably be considered cycletracks. Starting from the west, the section along Seaview is a two-way bike path paralleling a major road, but doesn't have many intersections and abuts a ridge to the east. The section along Northlake near Gas Works Park, however, fits the formula. It runs partially at sidewalk level and partially at street level, between the street and its sidewalk, which abuts actual buildings. Sections in Wallingford and the U District parallel Pacific but typically have enough vegetation or topology separating them from buildings that they don't really feel like cycletracks. Farther north and east there are similar sections of the BGT and Sammamish River Trail that aren't in Seattle proper.
The path between Harbor Ave SW and Alki Beach is certainly built like a cycletrack in places. However, when the weather is nice it's so packed with people walking and skateboarding that people biking are pretty much forced out to the road. One might say something similar about the path east of Alaskan Way, except that it's unusable even on rainy days.
The Duwamish Bikeway parallels West Marginal Way, running between the road and riverside industrial land. That gives it a case for consideration as a cycletrack, but not an airtight one.
So where do we go for real cycletracks? Renton, of course. Renton? Yes, Renton. The city that slapped down an asenine 10 MPH speed limit on the Cedar River Trail first built it as a cycletrack along Maple Valley Road. Renton also has cycletracks along stretches of Logan, Garden, and N 8th St; though they don't exactly form a complete network they're sort of useful, and aid bike mobility in that part of town a bit.
So now we see: history and bike infrastructure didn't begin yesterday! Several cycletracks (or, if not, fairly similar types of paths) have been built to fill in gaps in routes or networks. Linden follows in this tradition, like Ballard's “Missing Link” will. It's primarily a through route so it isn't like the Broadway Cycletrack, which I think really will be something new for Seattle: a cycletrack built for local access. Still, when Linden is finished it will feel significant. It will be the longest path that's unambiguously a cycletrack, has adequate parallel sidewalk capacity, and has regular intersections. I guess that counts for something.
EDIT: Naturally I missed one. The bike path in the median of Beacon Way could be considered a cycletrack. I'm not sure boulevard median paths aren't inherently something different, but it has a case. It's a very flawed path, but if a cycletrack can't be flawed I don't know what can be.