In this paper a computational model developed with the objective of simulating the thermal behaviour of the passengers' compartment of vehicles is presented. The model is based on the space-integral energy balance equations for the air inside the compartment and for the main vehicle bodies and surfaces. It can solve two kinds of problems. In the first one, calculates the heat stress that the air conditioning or heating system must equilibrate, in order to satisfy predefined permanent regimen project specifications. In the second one, once imposed a particular air conditioning system and given the ambient conditions, it computes the different temperatures and heat fluxes either in transient or steady regimens.The validation of this model was done with a railway car, in a summer situation, when it was immobilized and running. The model reproduced well the experimentally determined temperature and heat fluxes evolutions. However, the numeric simulation showed a best agreement with the experimental results when the convection heat transfer coefficients, determined experimentally in this work, were used.