You can for a lot less than $1bn/mile. Crossrail was ~£200m/mile. Even the Channel Tunnel, under the sea, was £300m/mile. Those prices are all inclusive, so tha

You can for a lot less than $1bn/mile. Crossrail was ~£200m/mile. Even the Channel Tunnel, under the sea, was £300m/mile. Those prices are all inclusive, so that includes building stations, rolling stock etc. Also unlike old cities, there are no archeological issues when digging under LA.