Hi Steve,
In case you missed my previous resonse to this question:
This is about the only shot that I have down below the high line bridges. This was taken from forest road 625 shortly after turning off of highway 145. As Tom mentions, hiking the grade near the high bridges is challenging. I happened to drop down between two bridges from the highway above. Hiking back up is steep and was tiring for me at that altitude. I didn't even want to think about trying to find a way across the sites of any of the spans. As Fritz mentioned there is private land up above bridge 46F. A dirt road leaves the highway up there that is posted and has a gate.
Above Matterhorn the grade crosses the highway and is a road. We drove the road (former grade) from Lizard Head pass and then around Trout Lake to Matterhorn. A few years ago the portion between Lizard Head and Trout Lake wasn't open to vehicles. I'm not sure when it was opened back up.
I have updated the map to show the approximate location of the "loop". The link is provided below if you would like to take a look. As you can see from the map highway 145 crosses the loop and has destroyed some of the grade. You also have the forest road which is on part of the grade. as you can see from this photo it looks like some earth work has been done in the foreground. Maybe it is part of a super fund site. Looking at the aerial photo you may be able to see the most eastern part of the loop after it crosses the highway. I didn't photograph this or try to determine if it is actually the grade.