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That $1.6 billion was my back of the envelope estimate based on costs I found in the news. Apparently, there is a unnamed source quoted in the Tribune that the cost would be under $1 billion. Also, these skates are the width of an electric car, so one tunnel 28 feet wide should suffice for bidirectional traffic.

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If your goal was to cut costs and your largest single expense was boring, you would choose two 14' bores over a single 28' bore, because a 28' bore has a cross section twice as large as the combined cross section of two 14' bores and variable costs for boring are roughly based on cross sectional area

One of the only things that I can agree with Musk in regards to this project is that larger tunnels are more expensive.

Wouldn't you need a third tunnel for emergency escape / access?

If they can really do it for that price, that’s impressive. In London one of the tube lines is being extended by 3.2km and it will cost $1.6b (£1.2b):

https://www.london.gov.uk//press-releases/mayoral/nle-tunnel...

London is relatively easy to tunnel in, and as it’s an extension existing rolling stock will be used (so no development or purchase costs).

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