High-speed rail is often judged by whether ticket revenue repays construction costs, a test almost no line passes. Advocates argue this is the wrong ledger: the benefits that matter — decongested roads and airports, connected labour markets, development around stations — accrue to the wider economy rather than to the operator. Critics reply that such diffuse benefits are easy to claim and hard to verify, and point to lines whose ridership never approached forecasts. Both sides agree on one thing: the case for any given line depends heavily on the density of the corridor it serves, which is why identical technology succeeds in one country and struggles in another.