This is one of the best mountain trails in Yilan. There are actually four trails that meet in Taoyuan Valley 桃源谷. Each one is quite different. The so-called Daxi branch of the trail begins at a spot just north of the town of Daxi 大溪, not far from the Honeymoon Bay surfing beach. The trailhead is just 800 meters north of the Daxi train station, so you can easily get here by train from Taipei. (Just make sure you go to the right Daxi and don't end up in Taoyuan county.)
The trail is easy to find. There is a large parking lot at the bottom marked by a conspicuous brown sign. In fact the entire trail is very well constructed and maintained. It is lined with large paving stones the whole way, and occasionally you will come across a stone bench or pavillion. Several large maps along the way will let you know where you are, and milestones mark the distance along the trail. There are even restrooms. It's impossible to get lost - you just follow the stone path onward and upward.
This trail is moderately challenging, so you shouldn't attempt it unless you're in decent shape. You don't have to be an athlete to finish it, but it's 10 kilometers round trip and you're walking up steep stone steps much of the way, so you'll be really tired when you're finished. The trail takes at least 4 hours round trip - longer if you linger or you're a slow walker.
Nevertheless, this trail is well worth the effort. Along the way you pass through a series of varied landscapes and microclimates. At the bottom the vegetation is sub tropical rainforest. Then you pass through low mountain vegetation with great views of the sea and Guishan Island. After that you enter the mountains, and there are impressive green mountain vistas on either side. Then you pass through a series of lush fern forests. Finally you find yourself in an idyllic alpine landscape at the top. The highest point is about 550 meters above sea level.
Besides being a living encyclopedia of Yilan flora (including lots of wild rhododendrons and some strange mushrooms), there is also some fascinating animal life - besides the usual butterflies, lizards and birds, I also saw a beautiful green snake and two muntjacs.
The first 45 minutes of the trail are the hardest. You're climbing up steep stone steps almost the whole time. After you reach the top of the first peak, you walk up and down along mountain ridges the rest of the way, so it's much easier going. At points the trail goes through extremely humid rainforest where the stones are covered with very slippery moss. Be careful! In these places it's a good idea to avoid the mossy stones and just walk on the dirt beside the trail.
If you're a glutton for punishment, there is an undeveloped dirt trail which runs beside the main stone path. As you walk up the stone steps at the beginning, you will eventually see an unmarked trail to the right leading off into the woods. This is an alternate trail for those who want to walk along the forest floor.
You will come across some stone barriers as you approach the end. These keep feral water buffalo from wandering down the mountain. The buffalo aren't domesticated, so be careful. Keep your distance and don't startle them.
Follow the trail until the stone path comes to an end. Here you will find yourself in an amazing alpine meadow, lazily observed by some grazing water buffalo. This is the crest of a ridge above Taoyuan Valley. It's hard to believe you're right above a subtropical rainforest - the scene seems more like Switzerland than Taiwan.
The Daxi trail meets the Caoling branch of the Taoyuan trail here. If you want a longer walk, you can keep going along the Caoling branch. A round trip back to Daxi would be 18 kilometers and would take all day. Alternatively, you could follow the Caoling branch of the Taoyuan Valley trail to the Old Caoling Trail and then either walk down the other trail to Fulong or back down into another part of Yilan.
Because this trail is more challenging than most, unless you happen to run across a hiking club you don't have to worry about crowds. I went on a weekday and didn't come across a single person.
The temperature at the top is much cooler than the bottom. I climbed up on a hot August day, and it was surprisingly cool and misty at the top. If you go any time of the year but summer, be prepared for cold conditions on the peak - you will need warm clothes. And since this is a 10 kilometer walk, be sure to bring lots of drinking water.