The article presents an adverse impact of the transport in agglomerations on the environment and climate conditions. The sources and acceptable levels of environmental pollution and climate degradation are presented as well as contribution of the cars to this process, together with the actions undertaken with a view to reduce the emission. The trends in modernizing of the public transport systems are pointed out, inclusive of justification of the ecological tramtrain system implementation. The next part of the paper includes comparison of the features of the tram and passenger rail vehicles used for public transport in agglomerations. A brief outline of the actions initiated to implement the tram-train system in Poland is depicted. The final part of the article highlights the technical issues related to the difference between the tram and rail vehicle construction and equipment and discusses the scope of organizational and legal tasks necessary to undertake the practical measures. The Polish contractors potentially able to perform these tasks are indicated. In the last part a brief assessment of railway network suitability for implementation of the tram-train system was carried out on the example of the Poznań agglomeration. An extensive bibliography makes an essential part of the paper. The present article is a continuation of the earlier papers developed with participation of the author. Some of these problems have been only identified in the former papers and, therefore, they are discussed here more clearly, due to their importance to the topic of the tram-train system. The second part of the article will be published in the next issue of the quarterly Rail Vehicles.