Seattle - Everett Interurban Trail
This is an interesting ride that takes you from 110th in Seattle to 41st in Everett. Some of it is on streets and there is an element of route finding until you get to about Lynwood (and then again in Everett north of the mall.) There are long stretches, though, where the pavement is good and smooth and where there are very few street crossings. Seattle and Shoreline are spending a lot of money upgrading their segments of the trail and it will be really posh once they're done. Edmonds is at least theoretically starting work on their segment and there are developments in a couple of other places that will expedite passage. The State DOT is building at least one bridge across I-5 (although without any connection to the trail yet.) and within a few years the route ought to be straight forward and pretty much contiguous.
For now, expect detours around electric substations blocking the path. Expect traffic around Alderwood Mall, a bad freeway crossing next to the drive-in theatre, and on-again off-again trail in Everett. It's worth doing and it connects to a lot of other stuff. Here is an update on the status of the trail in Shoreline.
Here is a link to a history of the Interurban Trolly line.
Here is a link to the Snohomish Country Interurban trail brochure and map. Here is an article about a new bridge for the Interurban to be built over I-5 (and here is an update .) Here is a link to the proposed 7 mile long North Creek trail in Mill Creek that would link the Interurban trail to the Sammamish waterway trail (although not until 2007.)
Here is a link to a map of the route done in TOPO!. (Be aware that this is a very large file)
Here is a link to a gallery of photos taken of the Interurban trail on a rainy day.
Missing links in Mountlake Terrace and Edmonds:

(Here is a map of the area with the Edmonds segment - it's a large file for a short segment.)
Here is a link to the Shoreline Inturburban trail map. Here is a link to an update on the status of the new Auorora bridges in Shorline.
Here is a link to the Seattle Interurban trail page on their website and a copy of their map. Here is an email saying that they plan to start construction in 2005.
All the way north is the Bellingham Interurban trail