It's interesting you should mention the overhead wire. That reminds me how proper metro systems normally use third rail, not overhead - for the very reason that it allows smaller diameter tunnels to be bored, reducing costs. Of course, with a third rail, at-grade intersections are impossible. I wonder if the RPA ever weighed the higher cost of proper segregation against the reduced costs of tunelling at a lower diameter. It might just turn out that a real metro is even cheaper to build.
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