This is a great place for parents to take their kids to explore a fascinating part of our history. Both the west portal to the Old Cascade Tunnel (2.6 miles long) and the massive concrete snowshed over 2,000 feet in length are close to the trailhead with many informative displays about the history of Wellington, later Tye, and the avalanche disaster that happened here almost 100 years ago. Information on the avalanche is a short distance down the snowshed. You may also continue through the snowshed and hike the Iron Goat Trail. The drawing below, courtesy of the Monroe Historical Society, is of the switchbacks used to route the Great Northern trains to Wellington from the summit of Stevens Pass from 1893 until the First Cascade Tunnel was built here.
Park at the Wellington Trailhead near Stevens Pass. Directions on how to get there can be found here. You will also need a Northwest Trail Pass.
For more information, visit the Iron Goat website and the Wellington Avalanche website
You are looking for a regular lock'nlock box under the bridge at the entrance to the snowshed. Try looking behind the fifth vertical support from the east end on the north side.