If you are looking you will see RR related items here and there. Dave is correct about bridges and tank base footing. There remains the small bridge as well as the tank base footing at Tank 7. There are buildings still alive at Sargent too. There are subtle remnants of buildings (foundations at best) atop the pass and the turntable pit as well. Some snow fences on the west side remain. The Marshall Pass drive is very pretty, very serene and quiet (more so on the west side). Nice camp spots if you were planning on staying on the pass somewhere. Mt. Ouray is a nice hike depending on your hiking ability/interest.
The Alpine Tunnel road is very mild and you will have zero trouble in your Tundra, at least if approaching from the west (Parlin/Pitkin) side. (The road over Hancock Pass from St. Elmo is more difficult.) The only place that is narrow is on the actual Palisades but you can see if there is someone coming from either side before you head up or down and isn't rough or too narrow. You will be fine.
Both drives are well worth the time and camping in the area is wonderful.
Ross-