The basic idea is simple: The single biggest cost of any metro system is the stations, whose cost scales with size. Therefore, if we run a system for smaller trains, we can build smaller stations for these trains, saving a huge amount on station costs. This costs us in reduced total capacity, but this can easily be made up for by increasing train frequency.
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Capacity limits are why we didn’t always do this: Train operators are expensive, which limits the number of trains you can run since it makes it more efficient to run fewer longer trains. It’s the invention of fully driverless modern trains that makes this obsolete, making running more trains basically free. But now that we can run fully automated trains, the only reason to run longer trains is if we’re hitting hard capacity limits after maximizing frequency.
This isn’t impossible - it happens in Hong Kong, and can easily happen in other dense places like Manhattan - but it’s uncommon. [...]
From the article:
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