If you lookup the details about the decision to build and construction of Stuttgart 21[1] it's an insane mix of corruption, nepotism and incompetence. Partially also a result of laws changing and allowing privatisation of public infrastructure.

This satire [2] about it on German TV is 6 years old now and the project is still increasing in cost and being delayed. It's a pit without a bottom now.

[1] https://en.wikipedia.org/wiki/Stuttgart_21

[2] https://youtu.be/V49b13fYFik