Those cost increases are not unique to Cologne though. It starts with a cost estimate that’s way below a reasonable sum of money and that is highly unlikely to work out, but one that is politically feasible. Then (once planning and building is happening) costs increase and often enough, public sector decision makers are inept at damage control when unexpected things happen (which they do) and plans need to be reworked accordingly. Sunk cost fallacy is also real. Things then get out of hand and construction companies know that shit needs to get done at some point and there’s more money to be made once the child has fallen into the well.
You’re not gonna be able to start a 600 million euro project if you’re honest about it. You say it’ll be 250 million, get it greenlit, and go from there. It’s stupid, but that’s how it works everywhere. I’m not saying that there is no corruption involved, but I don’t think it’s the worst problem. Look at Stuttgart 21: It is now at least 5 years overdue and cost increased from 4.5 to 11 billion.
Köln is indeed special.