Please read the actual link. This simply isn't an issue the way you think it is. "It's the unions" is simply being intellectually lazy because it fits your preconceived notions.
"I am not overlooking union power, I believe it is not an important factor, not when right-to-work US states have very high construction costs too while Scandinavia has low costs."
The problem is the quantity of labor. In the US we use many more people to operate a TBM vs what they'd use in Europe. On commuter rail lines we pay conductors $100k a year to collect tickets when in Europe they'd use a ticket machine. On the NYC subway they have a driver and a conductor when in Europe they'd only have a driver at best, or even some lines are being made driverless I believe. They're trying that in NYC too but it's complicated and costly because of how old the signaling system is, because it was too costly to ever upgrade.
The agencies should focus less during union negotiations on keeping salaries and benefits down and more on reducing the quantity of required workers. If they were able to do it in Europe, they can certainly do it here.