I'll take a stab at this.

I think the NP line from Snohomish to Arlington was abandoned north of Hartford around 1972. Hartford is a few miles north of Snohomish. I believe that segment (from Snohomish to Hartford) was abandoned in 1987(?)

If you map quest Hartford, you can see the Centenial trail along, what used to be the rail line. There are a couple businesses that obviously were the reason the line existed to Hartford beyond 1972, however I don't recall what they are specifically.

The line into Snohomish proper remained to serve a feed mill, which was torn down a few years ago to make room for the new public library (since completed). Initially, the City was going to remove the rails which did remain (approximately 1 mile), and extend the Centenial trail to the City's river front. However, after removing all crossings (of course) they decided to leave the rails, to possibly operate a trolly. Thus the Centenial trail runs along side of the abandoned roadbed. There was some talk that the Spirit of Washington dinner train would use the line into the City (requiring the crossings to be put back in), but I've read bits elsewhere that the dinner train would stop at a point south of the Snohomish River bridge on the Woodinville branch.

Cheers,

MCL