Someone who looks like me has ridden and driven through the Donner Summit snow sheds many times in the past ( it has always been illegal trespass on RR property and Southern Pacific takes this seriously so I admit to nothing). Really a lot of fun with great views over Donner Lake through the periodic ventilation windows. Good rock climbing and granite scrambling opportunities too. Various artifacts from Chinese pottery fragments to left-over iron oddities can be found outside the tunnels on the exposed slopes. The coarse rail road ballast on the roadbed can be annoying, being all about the same size it can be like riding of golf balls.
However last time there the uphill tunnel exit at the Sugar Bowl ski resort parking lot had a blocking locked wire gate. There is limited, but scenic riding on the eastern downhill Cold Stream side but being so close to Tahoe one encounters private property holdings where legal use of the roads is questionable.
Still lots of regional dual sport riding can be done. Simply riding the old highway route from Donnor Lake over the Rainbow Bridge to Sugar Bowl should be added to anyone's visit.
The abandoned snow sheds that protected the trains from avalanches can be seen on the far canyon wall around the 1:20 minute mark. The original transcontinental railroad grade here was replaced with extensive continuous taller tunnels once double-stacked trains became the norm. I always though S.P. should sell the old alignment to the government as part of the Rails-to-Trails Program as the avalanche tunnels are safe, scenic, accessible historic testaments of the dramatic settlement of the west in the 1860s.