Nag, Brandel-Tanis, Pramestri, Pitera and Frøyen provide one of the most useful recent reviews of digital twins in transport planning because they separate operational promise from conceptual inflation. The iconic idea is that many so-called transport digital twins are in fact digital models or digital shadows lacking real-time data integration and bidirectional interaction with physical systems. Its theoretical contribution is a maturity critique: digital-twin value depends not only on technical sophistication but on the spatiotemporal scale, service component and planning usability of the system. Methodologically, the review follows PRISMA procedures and analyses 136 studies from 2000 to 2024 through an adapted five-component definition: physical, digital, data model, service and connection. Its conceptual operation is diagnostic classification, clarifying what a usable twin must actually contain. It bridges smart-city discourse, transport modelling, infrastructure planning and STS by showing that digital twins are planning instruments only when their services, feedback and public functions are explicit.