That's an apples and oranges comparison, because one happens to already be built and the other does not.
Those are sunk costs only for the lifetime of the materials. Then a total reconstruction is needed, usually a paving every 10 years and a total reconstruction every 40-50.
Once the rail tunnel is built, it will be a sunk cost too. And there is no record yet of a rail (or highway) tunnel needing to be rebuilt once constructed.
I guess replacing useless concrete ceiling panels couldn't be considered "rebuilt" either.