So here's a topic that no one seems to have posted yet -- tunnels. As I was sifting through my collection for other threads, I realized I had a handful of interesting cards on this subject that were begging to be shared. And I know there are more out there.
A couple river tunnels -- the Hudson River subway tunnel going into a curve and the Detroit River rail tunnel, headed for Windsor, Ontario.
Southern Pacific locomotives heading through the Sierra Nevada range had to tunnel through snow sheds designed to spare the tracks from avalanches.
The Mitchell Point Tunnel was one of the iconic features along the historic Columbia River Highway, with its carved "windows" and view of the Gorge. Sadly it was blasted away to make room for Interstate 84.
The Wawona Tree tunnel was opened for through traffic in 1881, well before Sequoia National Park was established. It stood until 1969, when a heavy snow load brought down the tree. It can still be seen in the park, but is now known as the Fallen Tunnel Tree.