: This paper reviews the contribution of experts to the appraisal of large infrastructures, looking at the evaluation performed in 2019 in Italy by the Ministry of Infrastructures and Transports on the Lyon-Turin railway project. This evaluation had been initiated in a particular context, linked to a visible policy agenda and while the construction of the tunnel had already begun. The appraisal has produced surprising results, mostly counterintuitive and heavily criticized. We analyse these results and show how they can be valid, in that they would not result from inconsistency in the method but from the peculiar features of the project (low initial demand, high mode transfer compared with initial traffic, high road tax and toll levels). We also check the validity of some paradoxical results of the assessment. Moreover, the public debate has been made more difficult due to an inconsistency in EU evaluation Guidelines. Also, we analyse mechanisms that affect the credibility of experts and we find that the influ[...]