I spent yesterday on the Whitehorse Trail, hiking from Swede Heaven to Darrington. Trail in this section is in great shape, no issues, no obstacles. Be aware that parking at the trailhead is extremely limited, room for maybe three cars. I read that one can park at the junction of SR530 and Swede Heaven Road, It is about .5 miles from there to the trailhead.
The hike to Darrington is a good one, following what I believe was once an old RR line. There are ample spots to view the Stillaguamish River, with some limited river access. There are also a few creek crossings, bridged, about a mile in from the trail head. One does offer creek access, and I spent a little time there. I never could find the name of the creek. There are also many views of Whitehorse Mountain along route, making this hike a nice one. When you reach the amphitheater area, stay to the right on the line of trees, bleacher area. it is much easier to pick up the trail again on the other side. I did not go all the way to Darrington, passing the lumber yard/area, and turning back at this point, about a half mile short of Darrington.
When finished, the Whitehorse Trail should be a great one. I hope to return and hike as much as possible from the Arlington end. The trail should be accessible year round. I can imagine that this would be a good winter hike too.