Head up Abbot Creek Rd about 3.5 miles to the campsite on the left side of the trail. Just past it there is what looks to be a singletrack to the left. This is the start of Shay Railroad Trail. While timid at first, it has sections that drop extremely steeply with loose sand and deep ruts. I do not recommend doing this solo. In a bit more than 1.5 miles, look for a marked left turn option or continuing straight is referred to as "Goodmill".
If you're looking for a downhill challenge, Shay Railroad is it. Be aware, there is a fair amount of climbing on Goodmill to get back to the Millwood staging area.