The Green Line, with its many branches, ends up the lowest of the lot at just 10,087/milepic.twitter.com/XwRlmby6PH
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...so, with that in mind, I think we can make a good guess where the break-even point is for a generic subway line: roughly 15,000 per mile
A subway line with higher passenger density than this should (all else being equal) be able to operate at a profit. If it can't...?
...time to start auditing!
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