One of the most common motivations for public transport investments is increased capacity. However, appraisal methodologies for projects meant to increase capacity are relatively less well developed compared to methodologies for projects aiming to reduce travel times. Each of the consequences of capacity limitations - crowding, risk for denied boarding and unreliable waiting and travel times - can increase the generalized travel costs. The appraisal of capacity improvements requires supply and demand models able to capture the processes that lead to uneven distributions of vehicles and passengers and monetary valuations of e.g. crowding, delays and unexpected waiting times. This paper integrates these building blocks into a comprehensive framework for appraisal. A case study of a metro extension that partially replaces an overloaded bus network in Stockholm demonstrated that congestion effects may account for a substantial share of the expected benefits. A cost-benefit analysis based on a conventional static model will miss more than half of the benefits. This suggests that failure to represent dynamic congestion effects may substantially underestimate the benefits of projects primarily designed to increase capacity rather than reduce travel times.