I’m not sure they were at all implying that California was exceptional in that regard, to be fair, but at least that urban tunneling involves many costs besides the physical boring.

In any case, I wouldn’t use the UK as an example of tunneling efficiently. Crossrail, for instance, is on the order of more expensive urban US projects and there’s a fairly consistent recent history of either not building or letting existing rail infrastructure decay.

This link provides a decently contextualized comparison IMO, although it is slightly out of date now: https://pedestrianobservations.com/2011/05/16/us-rail-constr...