"above ground" is a confusing term. I think the intent from
@mjl08 was "on-ground" and not above it.
There are a few factors such as station length and station spacing. Assuming 1km spacing, in today's dollars, I'd guess.
Tunneled using TBM (20m deep stations with 100-150m long platforms) = $400 to $600M/km.
Cut-and-cover (10m deep stations with 100-150m long platforms) = $250 to $400M/km
Elevated (80 to 120m long platforms) = $150-250M/km
On street (50 to 80m long platforms) = $100 to $125/km
Summary,
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