The interurban trail in South King County is not interesting or particularly scenic – it is straight as an arrow between the railroad tracks and the back of various warehouses from I-405 in Tukwila down past SR 167 and SR 18 to 3rd street in Algona Pacific. It is almost perfectly flat, though, and the part in Kent makes a good loop with the Green River trail.
Actually, I take back the comment about scenery. There are a lot of little wetlands next to the trail and at least in the morning you see a lot of birds and rabbits and stuff. There are a few at-grade street crossings (212th and Willis are the only ones with lights) but you can make pretty good time on it. There are several places where railroad tracks cross the trail but they seem to be sidings and they have rubber spacers so that they don’t threaten to grab your wheels. (You still need to watch out for them when they are wet.)
Evidently from 3rd the right of way hangs a right and heads over to Milton, Fife and eventually Tacoma. (This link to Edgewood gives a good description of where things are right now, here’s a snip from their application to PSRC for design & construction funds that gives an interesting overview, and here’s a huge PDF of the master plan for the interurban in Edgewood.)
On the other end the Duwamish trail trail follows the Interurban route almost to the Seattle city limits – and then takes you into downtown Seattle along the east side of the waterway. From downtown the interurban ran to Freemont and Ballard before heading up Freemont Street. There are a couple way to make the connection but probably the most authentic would be to leave the Duwamish River trail on interurban avenue in Tukwilla (by the Boeing bridge) and follow that to Airport Way. Go through downtown on Alaska Way or Second and take Dexter up to Freemont. 8th has bike lanes that take you up to Holman and Greenwood takes you to 110th. Cross on 111th to Freemont and the beginning of the Seattle portion of the Interurban North.