This hike is actually a loop around the entire lake. It was closed for a long time while they did seismic upgrades on Ruskin Dam-- it should be open now-- check first! I found the Reservoir Trail less scenic, and it is longer as well, so some may want to stick to the in and out of the Railway Trail :)
From the Website:

Follow the path of a historic railway

Railway Trail runs down the west side of Hayward Lake. The pathway of the trail once served as the railway connecting nearby Stave Lake Reservoir to the district of Mission. The flat and wide area makes for a fairly easy hike with a brief section of inclines and declines. Hikers and bikers can make their way down the 6-km trail all the way to the Ruskin Dam located at the south end of the lake before crossing over the dam to Reservoir Trail.

See the dam and waterfalls

On the opposite side of the lake, Reservoir Trail will take you on a 10-km walk through the forest. Follow the north end detour for views of Steelhead Falls and don't miss the canoe landing towards the south end to take a short break and enjoy views of the lake.