On the other hand, unrealistic cost estimates end up with huge overruns. The costs are intentionally minimized in official statements in order to maximize the buy-in then only later does the truth come out. Makes for skeptical people. Sure there are unexpected obstacles - but most of them are known possibilities or even probabilities.
For the most part you are just flat wrong. There is a considerable effort made to include reasonable probabilities and possibilities. Sometimes the politicians choose to adjust numbers for their own purposes. That cannot be stopped by the engineers and estimators. Just a thought: It was not the designers or builders of the Titanic that claimed that the ship was unsikable, nor even the operating side of White Star Lines. They knew better. It was the publicists and politicians that came up with that nonsense.
The poster child for your statement has been Washington DC Metro, and to a lesser extent MARTA. What everyone using these as examples is the runaway inflation we were dealing with between initial cost estimates and construction.
Conversly, the first round of DART, because the estimates were realistic the propoganda you just presented was used to kill it for several years. Then when construction actually started quite a bit came in
under the estimates that were made some 5 years previously.
There are a lot of individuals, political and other agencies that are doing their best to make sure that they get things added to drive up the costs of the HSR system that will then with great joy and enthusiasm be amoung the first to blame those that were trying to keep things reasonable.