[T]unnels and railways, these can last for decades. For example, parts of the London Underground and Manhattan Subway are over 150 years old.
The NY/NJ Path tunnels—the first ever built under the Hudson River—were completed 100 years ago in 1908, and are still in use. Construction on those tunnels, started in 1874 by a private company, cost $50-$60mm in 1908 dollars (~$1 billion) and required lots of creative financing.
Exactly! If you really think about it, the people who will be working and paying the bonds off in 2057 will probably be benefiting so much from the transit, probably more than those working right now, and will be happy to pay some of their share.