Boston's Big Dig (1991–2007)
Boston's Central Artery/Tunnel project rerouted I-93 underneath downtown, replacing an elevated highway with a tunnel network. Originally estimated at $2.8 billion with completion by 1998, it ultimately cost about $24 billion including financing and was substantially complete only in 2007.
The project finished roughly nine years late and at nearly nine times its original budget, becoming the canonical American example of megaproject cost overrun.
Boston ended up with the infrastructure it paid for — the elevated highway is gone and downtown is reconnected to the waterfront — but the political fallout made future federal partners more cautious about large urban infrastructure commitments.
Like California high-speed rail, the Big Dig demonstrated that American megaprojects routinely run multiples over their original cost and decades past their original timeline — and that the underlying infrastructure can still ultimately be built and used.