Using data from a stated preference survey conducted in the UK, we show how the relative importance that rail travellers attach to reductions in greenhouse gas emissions and faster journey times varies strongly as a function of underlying attitudes towards the environment. We specify a latent class structure that allocates respondents to two classes with substantially different valuations of greenhouse gas emissions, and show how the allocation of a given respondent to either class is a function of underlying attitudes that also drive the answers to a number of attitudinal questions. We also show how these underlying attitudes are a function of a number of socio-demographic characteristics, with female respondents, older respondents, and respondents with a university degree having a stronger pro-environmental attitude, with the opposite applying to respondents with regular car access.