The Subway is a beautiful semi-technical slot canyon in Zion National Park. To complete the hike you wade/swim/scramble/climb down the Left Fork of North Creek, in the Kolob Terrace section of the park. The stream is the trail for most of the route. Some of the holes are deep enough that you do have to swim, and the water is cold.
The hike is somewhat like the famous Narrows, only more intense. You climb down waterfalls and scramble over boulders. Serious rappelling is not necessary but ropes come in handy to help get people over cliffs and to lower packs.
Most canyoneers hike this canyon from top down - it is very difficult to do it bottom up. A shuttle is required. You must have a permit to hike this canyon; permits can be obtained via the park website or at any park visitor center. Permits for this canyon go fast during the summer season, so apply early.
More details on hiking the Subway. Get detailed trail information before doing this hike. Only attempt this canyon if there is an experienced canyoneer in your group.