The measurement of the quality of transport services has been the object of an intensive research activity in the last decades. The EU project METPEX aims at advancing the state of the art in this crucial research area by focusing on current research gaps and less explored issues. To achieve this goal, a targeted survey covering the above aspects has been implemented in late 2014 in eight European cities. More than 6300 observations have been gathered through several different protocols, ranging from traditional face to face or self-administered surveys to gaming apps and tracking through a crowd-sourcing interactive navigator. This wide dataset is then analysed to find measurement models through the identification of latent constructs that can measure the quality of the traveller experience. In this paper, we concentrate on the identification of the underlying dimensions of perceived quality. These were elicited in a specific section of the questionnaire by pre-defining 21 quality aspects and then administering to each respondent a set of questions focusing on only one of such aspects. Our goal is therefore the search for latent factors hidden within such sets of questions that could more thoroughly represent the corresponding aspects. We apply a Principal Component Analysis to find the most important components and to see which quality issues hide latent factors that can give deeper information on the users' perception. The preliminary analyses presented in this paper are part of a wider work whose goal is not to just build a model from the data, but to identify those aspects of the service which influence the overall passenger experience. In fact, additional sets of indicators will be defined through the techniques used here for both public transport and active means, and for different user groups. Thanks to this work, a useful guidance to policy makers and stakeholders could be provided in order to encourage more virtuous travel behaviours with a more integrated approach