Waishuangxi Old Trail is one of four trails that climb from Pingdeng Village to Qingtian Gang, in the heights of Yangming Shan. Unlike most Yangming Shan trails it is largely unpaved, and unlike most unpaved trails around Taipei it doesn't feel like an obstacle course. It offers no views, aside from Qingtian Gang at the top, and no waterfalls, aside from Shengren Waterfall at the bottom. It is however pleasant to walk, and the Waishuang Creek is quite pretty.
When I did this walk I started off at Shengren Waterfall, which can only legally be viewed from a distance. (I climbed over a fence, across a stream and up to yet another fence to get a better look. The second fence warned of a NT$3000 fine and was adorned with a white bouquet, so I decided to stop there.) From there I walked up Zhishan Rd., past the turnoff to lane 371, and up the first set of stairs on the left. These stairs offered good views of the terraces and valley below. Eventually they led to the Old Pingdeng Irrigation Channel, which if followed north (right) leads to the Waishuangxi Old Trail. The channel passes an iron gate, but no one cares if you go in. Soon after the channel merges with the stream you have to clamber up the hillside on the left through ferns to find the trail, since a small landslide wiped out the point where the trail originally met the stream bed. From here it's pretty easy. Eventually you pass a right turnoff; straight ahead is the most direct way to Qingtian Gang. If you turn right you will climb up a tributary valley and eventually reach another fork: left for Qingtian Gang (remeeting the previous trail on the way) and right for Mt. Gaoding and Shitiling.
If you have Richard Saunder's excellent "Yangmingshan: The Guide", this walk follows the "easier route" og the Pingdeng Li Old Water Channel trail in reverse, meeting the Waishuangxi Old Trail at the end of that hike's point 1 (i.e., near the rest station). Just keep in mind that the first right is not mentioned; Richard instead describes the hike going straight.
If you don't have Richard's book, you should buy it now. It's the best guide to anywhere in Taiwan that I know of.
For a description of the trail in Mandarin, see this page. The map is also better than Richard's- it shows the turnoff mentioned above (with a question mark), as well as detailed descriptions of other features along the trail.
My photos can be found here, in my Yangmingshan album.
Shengren Waterfall to Qingtian Gang via Waishuangxi Old Trail
Time: 5 hours at a leisurely pace
Trail: Stairs for first hour; followed by an hour along a concrete water channel, followed by a slow then steep climb along a clear dirt trail. One short section of scrambling through underbrush.
Directions: S18 from Jiantan MRT to Shengren Waterfall (2nd to last stop). From Qingtian Gang the S15 goes straight back to Jiantan.
Features: Terraced farms, stream, forest, grasslands