Or turbojet trains[1]

[1] https://en.wikipedia.org/wiki/Turbojet_train

Or Ekranoplans[2]

[2] https://en.wikipedia.org/wiki/Ekranoplan

or flying cars, or jetpacks, or a million other ideas that just had "engineering problems" that were supposed to revolutionize transportation.

My favorite historical example of this is that of the nautical screw.

Orwell mentions it in one of his lesser-known essays when discussing the problems with arguing about economics(1).

>"When the nautical screw was first invented, there was a controversy that lasted for years as to whether screw-steamers or paddle-steamers were better. The paddle-steamers, like all obsolete things, had their champions. Finally, however, a distinguished admiral tied a screw-steamer and a paddle-steamer of equal horsepower stern to stern and set their engines running. That settled the question once and for all. And it was something similar that happened on the fields of Norway and of Flanders [referencing economic questions]."

1 - www.orwell.ru/library/essays/lion/english/e_saw