This paper describes the results of a research project that aimed to establish passenger values of crowding on public transport services in the Paris region. Qualitative research, stated preference (SP) experiments, and passenger counts and surveys were conducted to obtain such values. A simple method was developed to quantify the passenger benefits of specific public transport projects aiming to reduce crowding on existing lines. This method was applied in a case study to the regional rail (RER) RER Line E extension project. With regard to the value of crowding, the research indicated that the perceived disutility of crowding could be more accurately described as a constant disutility per trip than as a travel time multiplier. However, for ease of application often the multiplier formulation was preferred. When the value of crowding was expressed as a travel time multiplier, values were obtained ranging from 1.0 when all passengers could be seated to 1.7 for standing bus passengers when the vehicles reached their maximum capacity. Also for seated passengers, multipliers well above 1.0 were observed for (highly) congested vehicles (maximum value = 1.5 for bus passengers). These values were applied in a case study that estimated the effects of an extension of the regional rail line RER E in the western direction, partially running parallel to the existing RER Line A. This extension would reduce the current (very) high crowding levels on the RER A and B lines to more moderate levels and generate benefits of about €23 million per year.