The Big Dig also had significantly more issues to deal with, including actually tunneling rather than just digging a hole and then capping it, corrosion resista

The Big Dig also had significantly more issues to deal with, including actually tunneling rather than just digging a hole and then capping it, corrosion resistance due to being below the water table a few hundred yards from the harbor, tunneling under active trains and through old unstable landfills, etc. US infrastructure costs are far too expensive, the Big Dig was a catastrophe of a project, but it's not really fair to compare this project to Boston's.