I am taking this photograph at a spot approximately even with where the old caved-in tunnel exits the mountain. The view is down Goat Canyon toward Carrizo Gorge. ...And yes, that is my thumb marring an otherwise spectacular picture (mental note: It's remove thumb from lens then take picture, not vice versa. Doh!)
Those tooth picks making up the trestle structure are 16" square. I wonder how long it took to build? One interesting side note for those not hearty enough to explore the trestle in person, you can get a pretty good idea of it's scale by visiting the San Diego Model RR Museum in Balboa Park. One of the train sets is a replica of this stretch along Carrizo Gorge. It's worth a visit. The train setups are more intricate than you think.