Perhaps even more exciting than the actual Subway tunnel itself was this "Waterfall Room" at the culmination of the hike, past the tunnel. Getting to this hidden chamber required me wading through several pools of crotch-deep, freezing cold snowmelt water while holding my camera and tripod above me.
While it was a great testament to my manliness, let's just say I would not have wanted to measure my manhood after such a chilling exercise in immersion.

I was treated to some gorgeous light by which to photograph the waterfall with a long exposure. The Subway qualifies as a slot canyon, and slot canyons defy the conventional logic as far as lighting rules. Most of the time, the best photography is done early or late in the day, but in slot canyons, look for midday light to shine straight down and illuminate the walls of the canyon with a pleasant glow.