The ideal situation for personal transport in an underground city needs to consider two things: first, that the roads will be small, cramped, and probably shared with pedestrians, cyclists, and people getting around in non-automobile ways. And second, that emissions are simply a no-go, which restricts our options to emissions-free EVs.
However, vibes must also be considered, which lead me to my first thought: the 1970 Mazda EX-005 concept. However, while the vibes are immaculate—it looks like it was purpose-built for subterranean commutes—it's a rotary hybrid. Which, while certainly producing fewer emissions than a straight rotary, would undoubtedly choke out any subterranean city in a jiffy.
In the end, there is only one correct and good answer: the 1967 Ford Commuta

Electric, tiny, and cute. This thing could live in a city carved out of a massive subterranean geode no problem.
