Fuller's Traveling Cartridge
One of my new favorite blogs, Paleo-Future, shares this clever design from Buckminster Fuller showing "Fuller's Traveling Cartridge." That one would tour the countryside in a jaunty little carriage, only to load it Pez-like into an airplane or monorail for longer journeys, is optimistic in the extreme. It implies that every person would be willing to drive an identical car, with no opportunity to express their unique individualism via choice of automobile.
Plus that's a huge waste of airplane cargo space.
Fuller's Traveling Cartridge (circa 1960s) [Paleo-Future]

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Although, interestingly, I've seen futurologists predicting this sort of arrangement as recently as 2 years ago.
You could always decorate the outside of your cartridge with totally awesome stickers!
"My Other Cartridge Is a Porche"
"I'd Rather Be Jetpacking"
"Warning: In Case of Rapture This Cartridge Will Be Unmanned!"
"Impeach Bush"
I can see it now. Show up at the airport three hours early in your little cartridge, get in line for the TSA folks to come in and search the whole thing, send it through a scanner, then they can stuff you back inside and stick a TSA seal over the door that carries a $5000 fine and no-fly-listing for breaking it before landing on the other end. Safety through imprisonment!
AAAH! HELP! Lemme out of this box!
Looks a little optimistic on travelers' bladder capacity, among other things.