Simply using public funds to employ a workforce rather than contract one would save the most money due to eliminating the need to house and feed shareholders and turn a profit, but voters seem to like the contractor profiteering model even if it eats the public purse.
NYC only has a big subway system because it gobbled up all the failing private railways over the years. LA used to have the largest rail transportation network in the world, it was private, and it too failed to turn a profit and failed. Transportation runs at a loss.
Using public funds to employ a workforce is basically “How to Bloat 101.” The MTA in NYC is a shining example of taxpayer waste. They call it “Money Thrown Away” for a reason.
This is false. The biggest cost to tunnels is moving and replacing utility lines (power, internet, sewage, water, etc.) that were not properly recorded by private entities when they constructed their buildings.
It's precisely where government bureaucracy was involved that tunneling is the cheapest, because then moving and replacing the utility lines can happen all at once instead of over the course of months as the tunneling machine bores into them, and bureaucracy can coordinate road closures, utility shutoffs, etc.
Also, private entities use subcontractors not because bureaucracy requires it but because it lets them delegate out responsibility for areas in which they do not have competence for less than the cost of developing such competence in-house. It's simply the way the construction industry works. Even Joe Blow Contractor rebuilding someone's bathroom will hire contractors to do specific parts of the rebuild (like the plumbing, or the tiles). Another example: the SoFi Stadium in Inglewood has hundreds of subcontractors, and is fully privately funded.
Simply using private funds without numerous contractors can decreases costs by an order of magnitude with no technological improvements.