Preferences for transport activities are often considered only in terms of time and money. Whilst congestion in automobile traffic increases costs by raising trip durations, the same is less obvious in public transport (PT), especially rail-based. This has lead many economic analyses to conclude that there exists a free lunch by reducing the attractiveness of automobile transport at no (or little) cost for PT users. This article argues that congestion in PT - crowding - is also costly. Using survey data from the Paris metro we estimate the degree to which users value comfort in terms of less crowding. Using a contingent valuation method (CVM) we describe marginal willingness to pay over different parts of the distribution of in-vehicle crowding and consider moderating factors. We conclude that the total welfare cost for a trip rises from e2.42 for a seated passenger to e3.69 under the most congested conditions. We apply our results to the cost-benefit analysis of a recent investment in PT in Paris and consider broader implications for transport policy. In particular, we highlight that PT congestion is a first-order urban externality.