"But for a few quirks of fate - international depressions and world wars - it would probably be the main power source for getting us around."<p>No. The containment requirements for steam, plus recycling the water, vastly outweigh the same for the internal combustion engine.<p>Flight requires internal combustion, because the power-to-weight of steam is just to low, even though, yes, the raw power of steam engines is generally much higher than internal combustion.