As a car enthusiast, I can think of a good one, the Ford Nucleon.
During the 1950s and 1960s, there was considerable interest in nuclear power and its potential applications. This led to the idea of using nuclear energy to propel cars. The concept behind a nuclear car was to utilize a small nuclear reactor to generate steam, which would then power the vehicle’s engine.
Of course back in those days, this was extremely futurustic and some at the time thought this would be a game changer, but ultimately, the safety aspect was one of the biggest reasons why this idea was dropped, and I probably don’t have to explain why it may not have considered to be safe, I mean, it was using nuclear power, so even if the engineers tried to make it as safe as possible, IF something went wrong, it would have been catastrophic.
Ever since then, the interests in the automotive sector has shifted to Electric and Hydrogen.
Still, a very intriguing concept car and idea.
Outside cars, you have blimps, and I personally believe if we tried to make something like a hindenburg today with existing technology, we might have been a lot more successful than back then (as it goes way back to 1930s), there are still some blimps used occasionally, I also don’t believe those use hydrogen(?), but they are not the “game changer in air travel” it was once seen as, although we can’t rule out a comeback.
What about you guys?
You might enjoy the 1971 novella “A Meeting with Medusa” by Arthur C Clarke. It follows the captain of the last nuclear blimp as they become irrelevant, then later when his specialized skills are needed again. Giant blimps are used to explore Venus and the gas giants since they can’t be landed on and lag time makes remote piloting impossible.
This is interesting since blimps over Venus was recently in the news again for better or worse and is a relevant proposition. We will certainly have blimps over Venus someday (though probably not manned). The gas giants put out too much radiation and autonomous flight is too well developed for the Jupiter story to hold water now though.