This is the primary north-south route between downtown Seattle and Everett, easily branchable to Mukilteo and the ferry to Whidbey Island. Much of the route is on paved trail, with bike lanes and sharrows when the route continues on the road. There's a few tricky parts, but the signage is generally clear.
This route description starts from the Fremont Bridge; they are three approaches to the bridge from the south when heading out of downtown: via Dexter (which I believe is the 'true' Interurban route), via Westlake or via Nickerson.
From the bridge, ride the bike lane up Fremont Ave to 50th, turn left and in the bike lane around onto Phinney. Turn right at 77th (or any of the streets before 85th) and then left onto Fremont Ave which you follow all the way to 110th and the start of the paved trail.
Segment 1 - 110th to 145th [approximately 2 miles]
The first segment of trail starts at 110th, just east of Fremont Ave:
It runs from 110th to 130th and allows for fast travel and easier street crossings. Once you exit the trail at 130th, as shown below,
you will connect onto the very nice cycle track along Bitter Lake for the rest of the ride to 145th and the start of the second segment of trail, marked by a significant trailhead:
Here's the route map for the entire first segment:
Segment 2 - Trailhead (145th N) to Alderwood Mall (roughly 204th SW and I-5) [approximately 7.5 miles, 9.5 total]
From this point, the trail is straightforward and well-marked, and nearly all on paved trail, except for a couple portions in bike lanes. As you head north from the trailhead, you will cross two nice pedestrian/bicycle overpasses to get to the east side of 99. From there the trail heads north through Shoreline. The trail may appear to run out at 185th by the strip mall, but you can follow the sidewalk east along 185th to the first light Midvale Ave and pickup the trail again on the other side. It's well-marked. From here follow the trail to where it ends at 200th, near the Aurora Transit Center.
Here's the route for this section:
From there, follow the signs and ride the bike lane east on 200th up the hill to the intersection with Meridian. Cross here and pickup the connector trail on the north side. The trail sign can be a little hard to see. The entrance is only about 25 feet to the east of Meridian. Follow the short trail downhill to the busy intersection with 205th. Cross and ride the bike lane until the trail breaks off to your right with a clear trailhead. This newer section crosses 228th where the main trail picks up again. There are several street crossings as you follow the trail north to Lynnwood. As always, the route is well marked.
In late 2015, a spur trail opened that connects to the Montlake Terrace Transit Center via Lakeview. It runs east from the Interurban at 228th to I-5.
There is only one other slightly tricky portion before you reach Alderwood Mall - at 212th, the route twists and turns and continues on a small sidewalk through the South Lynnwood Neighborhood park. Follow it east and you rejoin the trail as it shares portions of the road before turning back into trail. There's also a small on-road section at 54th and 208th.
As the trail nears I-5th around 204th SW in the area of Alderwood Mall, the route to the Mukilteo Ferry breaks off and the Interurban North Trail continues over 44th and follows the west side of I-5. The wide pedestrian/bike bridge provides an easy way over 44th.
Here's the route for this segment:
Segment 3 - Alderwood Mall to Everett (Colby) [approximately 14.5 miles, 24 miles total]
Once across the overpass, follow the trail along I-5 past Alderwood Mall and then along 405; several stretches here are on bike lanes or sidewalks rather than true trails. After you cross under 525/405, you will be on Maple Road. It's a short climb over I-5 to the intersection of Butternut Rd where the paved trail resumes. Here's the intersection of Maple and Butternut, note the sign:
And here is what the entrance to the trail on Butternut looks like:
From here it's back on the trail for a stretch, the a detour over to 13 Ave W/Meadow Rd, and then you connect back with the trail. It's well-signed and easy to follow. Once on the trail along I-5, you follow it a mile or two before you once again hit a road, this time Meridian. Follow it around to 128th, then cross and head west on 128th back toward the freeway; the trail resumes right along the freeway on ramp. The trail crosses I-5 here and the trail resumes on the other side in two directions. Heading left (south) will take you back to 128th, heading right (north) continues the ride to Everett. Here's what it looks like heading south:
From here, the trail follows I-5 before veering off onto Mall Drive to bypass the Everett Mall. Follow Mall Dr. across Everett Mall Way; the trail resumes at the end of the road and immediately heads west to follow 526 to Casino Way where it crosses under and loops back to it's original line via a short stretch on Commercial Ave. From here it's easy to follow the signs for another 3 miles on trail and road, until the trail runs out at Colby and 41st: