In fact, there were a lot of projects that did have a real chance to become a part of nowdays life. Here are a few of good examples.
GM Aerotrain - promised to open new horizons.
Fairey Rotodyne - amazing hybrid of a helicopter and an airplane.
Schienenzeppelin - superfast train of early 1930's driven by an airplane propeller.
Mil Mi-12 - the largest helicopeter ever built.
A-90 Orlyonok - a soviet built ekranoplan.
What do you think about this Swiss passenger trailer truck?
If TTD had seaports I'd suggest the Spruce Goose.
These were actually built and operated. But how many other exciting projects were proposed? A LOT! Just start digging and you'll find enormous amount of intriguing stuff from jet trains to nuclear powered airships.
That would be great if someone added those marvelous designs in the game.