This 14 foot wide rail trail covers the entire 20 miles between Sedro-Woolley and Concrete, Washington, but the surface is too soft for my bicycling taste.  I prefer harder pack cinder trails, but Cascade is okay; especially for mountain bike tires.  It is a level railroad grade.  Probably more of a horse rider's paradise.  Another cyclist, I met, described this sight.  "Horses stopped by an expresso booth while their riders enjoyed coffee to go via horseback."
Read on below images.

Trail is located in Skagit County about 70 miles north of Seattle, WA.

Theslowlane.com.

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Wood plank surface employees old railroad bridges across wetlands.  The rest of the trail is soft in many ways.  Surface is almost too soft for riding.  Shade of trees  soften the sun.  A level grade softens any climb.  It's just a steady plod those 20 miles to Concrete.   I didn't ride the whole way on that trail at least. 

Smooth pavement of the quiet Lyman Hamilton Road was too tempting.  Visible from the trail, it beckoned me over.  Sauk Mountain is up ahead.

There's little traffic because the two towns are sort of "throw backs" to another era.  Off highway 20's beaten path and no branch campus of Microsoft nearby.  I can imagine the scene is retired loggers sitting in pools of cigarette smoke remembering the past.

The quiet road ended too soon near an area called White Horn.  Then I was dumped onto the shoulder of busy Hwy. 20, but it does have a nice wide shoulder.  I could have rejoined the trail, but smooth pavement continued to beckon me and time was running out. 

I camped, that evening at Rockport State Park a few miles east of Concrete.  No bike site, that I could find, so the fee was a standard $16.  State Park budget cuts have taken their tolls. 

Highway 20 is efficient, but busy.  It carries tourist traffic over North Cascades Pass.  This was just a weekend trip for me in 2002 so, next day, I headed back to Bellingham. 

Just before Concrete, 20 crosses "Birdview Pass."  It's only about 241 feet, if I remember the sign correctly.  A slight rise and someone, probably jokingly, erected a sign.  Skagit River Valley is flat.  This is a popular eagle watching area also. 

By the time one gets to Newhalem, it is still only around 640 ft.  From there the serious climbing begins as 20 crosses Rainy and Washington Passes.