Zero carbon footprint.
But only for the special people.
Like the special people in the US Congress
who really belong on the Moon for all the useless
“good” they do while groveling to Communists
and al Qaeda as they destroy Freedom.
Nothing has a “0” carbon footprint.
Seriously, everyone knows nature abhors a vacuum. Consider a structure 3000 miles long that needs to be hermetically sealed except for the ends, which need to open up large enough to accommodate a six-passenger vehicle, then quickly snap closed to make a perfect seal ... constantly. Sound feasible?
Remember it's outside, exposed to sun, weather, seismic action, so it will need expansion joints, but it needs to be perfectly straight, or straight enough that you won't spill a drop of your wine zipping through at 4000 mph...
Sure, sounds like a great idea - very do-able. That Keystone Pipeline, though, that's a disaster waiting to happen.