This article is focused on the freight aspect, but Amtrak operates a passenger Auto Train too. Danny Harmon has a great video about the unloading process: https://www.youtube.com/watch?v=fYlWBWlS4t0
This is quite common in Switzerland. You can stay in your car for some of them, which is a pretty funny experience.
Very few routes (maybe only one) offer this, however.
It's quite common for tunnels. Like the Eurotunnel or the Simplon tunnel.
Yeah I was saying that about Amtrak specificially.
Amtrak only has the 1 route, called the Auto Train. It's itself a single route, there are no others.