South of Princeville or west of Wailua on Kauai, the Powerline Trail is a popular 13 mile mountain biking route with awesome views of the lush forests. It is along an old dirt road cut through a wilderness-like setting in the 1900s for power line installation. Unfortunately, during the installation of new towers in the early 2000s, the majority of the route got really beat up and is heavily rutted.
It runs from Princeville in the north, along a southeastern route to a point west of Wailua. The route ranges from 650 to 2,000 feet in altitude. It is still occasionally used to service the electric lines. The route starting from Princeville is easy to intermediate. The route starting from Wailua is advanced due to a challenging climb right from the start. Therefore, many start the route in Princeville and head south for 8 miles to the ridge and then return via the same route. The first 1.5 miles are nice and climb gently. Then all goes to hell with a steep, washed out climb that is followed by a deeply rutted slick clay section. At best, the rout from here out is a hike-a-bike.
To reach the mountain biking trailhead from Princeville, head east on SR56 to Kapa ka Street (between milepost 27 and 28, and marked by Princeville Ranch sign). Head south on Kapaka Street for 1.8 miles to the signed trailhead near the large water tanks. To reach the southeastern trailhead from Wailua, head west on SR580 (Kuamoo Road) for 6.6 miles to the end of the pavement. Ford the river and continue up the dirt road for 0.25 miles to the trailhead sign on the right (yes, the trail is that extremely rocky, gnarly climb heading north).
Overall, Powerline Trail is the 7th most popular mountain bike trail of all 22 mountain biking rides in Hawaii.