Here's a link to the Centennial Trail page on the Snohomish county web site.
Part of this trail is on the grade of the old Monte Cristo railroad. Here is a book list.
The trail is really flat - like most rail-trails. It goes slightly uphill for about 16 miles from Snohomish and then there is a down-grade for the last mile. There is only one significant street crossing - and that has a light. There are several minor street crossings. There are gates or posts by each street and driveway. We counted once and found 36 pairs of barriers on the one-way ride.
The last time we rode the Centennial Trail there were cougar warning signs at the trailhead and on some of the barriers. We've never seen anything larger than a rabbit.
I expect that at the North end the trail will eventually follow the visible grade on down to 172nd where it connects with the sidewalk-style trail segment. It will be nice to see it continue into downtown Arlington and even better to see it go all the way to the Skagit County line. When King County talks about buying the BNSF right-of-way on the east side of Lake Washington they mention that it will hook up with the Centennial Trail. I presume that means that the South end of the trail will extend on down Maple (one hint is that there is a stub of a trail paved and signed between First and Second.) There is a high wooden trestle across the river at Cady park and then the maps show a railroad grade next to Springhetti and Connelly down to the Woodinville - Snohomish Road ending up in Wilmot Gateway park. Now that would be a ride.
Here's a link to an article about the new section and plans. And an update.