Wow, add me to the list - we should have an old-road-nerd party. 
Numerous random comments:
My university (OIT in glorious Klamath Falls OR) has a series of utility tunnels that connect all the various buildings, underground. When I was a student there, I was the student telecom technician, so I had access to a great set of keys that would open almost anything. We spent many an hour exploring the tunnels, finding the strange entrances (one door in the back of a closet which was in turn in the back of a storage room), observing as the tunnels reduced from standing-height to crouching-height to crawling-height, and then there was the one that just disappeared down into a pool of standing water!
On a related note, the Lava Beds National Monument (near KF, just across the border in California) is great for underground exploring. There are miles and miles of underground lava tube caverns that twist, turn, climb, dive and intersect. Even some "ice caves" that harbor frozen lakes year round!
Re: the 205 transitway - I used to ride that bike route from Oregon City to Vancouver as a kid, and I was fascinated by the 'ghost underpass' located near Holgate. I think it's been incorporated into the Max ROW now.
Scott re: the Mt Hood Loop hwy - I know exactly the place you're talking about. In fact I *did* follow the road, and it rejoins the new highway unspectactularly a few curves up the hill - if you know where to look you can see the spot while driving by.
You can also find (and actually drive on) the westward continuation of the old hwy on the other side of the new road cut that severs it - several miles downhill from Laurel hill there's a road called "Kiwanis Camp rd" that veers off to the North from the main hwy, which was apparently the old loop hwy right-of-way. You can follow it almost all the way to the point where the new hwy cuts through, and IIRC there's even a short stub that you can walk right up to the roadbed of the new hwy, complete with a small bridge. In fact that section of the old hwy is shown on Google maps!
Just west of Government Camp there's also a section that veers off to the north which is quickly blocked to car traffic but can be followed for some distance.
Further up the hill, after you turn off onto OR-35, (and near the "Pioneer Woman's Grave" sign IIRC) is a nice driveable section of the old loop hwy with a serious harpin turn. See here:
http://maps.google.com/maps?f=q&source=s_q&hl=en&sll=45.582507,-122.699567&sspn=0.013831,0.026093&ie=UTF8&cd=1&geocode=FfZhswIdA9u8-A&split=0&ll=45.280367,-121.704333&spn=0.013906,0.026093&t=h&z=16
There's also a section of the old Barlow Road near there that is car-accessible, and other remnants that you can trace through the woods. Neat!
Coincidentally I was just exploring the Orient drive and Bluff Rd area last weekend.
I'm not so sure about I-5 routing over the Morrison though... prior to the Marquam opening, what was then called the "Salem Freeway" would have routed onto Harbor Drive and eventually crossed the river on the Steel Bridge before continuing as Interstate Ave.
On a related note, I love looking at old roadmaps and I have a small collection I've sourced from Ebay.
Last thing (promise) - anyone ever noticed how rediculously oversized the Kerby Ave offramp from the Fremont bridge is(and how strange it is for a 2-lane demi-freeway to peter out to a stop sign)? This was apparently a stub for a never-built freeway through NorthEast Portland.
Posted on May 28, 2009 - 02:53 PM
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