I can think of a few reasons to run that slow.
- Running on a ledge high above the bottom of Toltec Gorge (note the guard rail)
- Entering a curved tunnel and not being able to see the other end
- Not too much clearance in there either
- See those rocks above the tunnel ready to slide down (after the train passed I heard some stuff sliding down)
- Plus you might want to let the passengers take in the view as the railroad was partially named after this location
The D&S runs just as slow up on the high line.