Not hard to reach, but it's quite a slippery hike to get there. In some instances, you'll have to wade through freezing waters to get there.

"The Subway hike Zion"
Featured snippet from the web
The Subway is a classic semi-technical slot canyon hike in Zion National Park. To complete the hike you wade/swim/scramble/climb down the Left Fork of North Creek. 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. - utah.com