Now that snow has finally melted up at the pass and we know what damage has been done after the long winter, I think it's time to see if there's still interest in doing this.
I'm thinking late September or October if the interest remains. Park near the tunnel, check out the westernmost tunnels and part of the abandoned Lincoln Highway. There will be a decent amount of walking at 7,000 feet and it will be dark/cold in the tunnels, but we can minimize the elevation change if people are up to that and stick close to the rail grade. Most of the cool stuff is right along the old rail grade or can be seen from it. Start early-ish to beat the crowds, maybe find somewhere in Truckee to have a group lunch once we're done. I might cross-post this with some of the history and Lincoln Highway groups given the subject matter, but if you haven't seen the tunnels and abandoned road, it is one of the roadgeek/railfan "things to see before you die".
A warning though: there's a rockslide on old 40 east of the Donner Summit Bridge, so the entire route cannot be driven. AFAIK, the road is closed just east of the bridge; this is something I'll need to check if I do another exploration visit before the meet. This would preclude driving the famous part of Old 40 after the walk, but most of it can be seen from the rail grade.
Exact date, I need to check events calendars before setting one, but provided there aren't fires causing smoke, late September/October is usually a beautiful time of year in the Northern Sierra.