I know express means non-stop, I'm just saying they don't even have 'express' trains from one part of London to another, so if you have an actual call for an express train then that is some big city you've got there.
Express trains (and busses) are also largely put in place to mitigate transportation needs explicitly around common peak hours.
Example: Lots of times you'll have cities where large numbers of people work in one area, live in another, and they all work roughly from 9-5. Most frequently that means that those express lines are usually just one direction in the morning and the other direction in the evening between those stops, and they just ignore other stops. That way they can go between the regular ones to supplement the regular schedule, rather than actually being separate "lines" or routes from one location to the other.
They're less about a city's overall size, and more about an analysis of its of population density, and specifically having a predictable movement of exceptionally dense parts of that population with an increased need for movement between a small number of locations within known times.
It's also worth noting that each Sector in Midgar is likely to have multiple stations that they can stop at along the way, and the Sector 1 to 8 Express line would likely just be hitting the major hubs of those places, and bypassing all of the other stops. Because of my earlier point, even relatively short travel distances can have express lines if they have the density to necessitate them.
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