Metropolitan underground railway transport systems are a very important part of the urban traffic since they carry millions of passengers per day in a number of cities around the world. They represent a segregated traffic microenvironment because of their closed character and restricted ventilation, lack of sunlight, specific emission sources and meteorological conditions. Concentrations of some air pollutants including aerosol particles are usually higher in underground railways than in corresponding external outdoor areas, which can considerably increase commuters' daily exposures. At the same time, the chemical composition and properties of indoor aerosol particles differ substantially from those for outdoor air; the particles are larger and heavier, are mainly composed of Fe, and contain less soot, and, therefore, their impact on morbidity and mortality is expected to be different. Nevertheless, some transition metals, i.e., Fe, Mn, Ni, Cu and Cr can be of concern. In the present paper we overview the physical and chemical properties and behaviour of the aerosol particles, as well as their major emission sources, compare the information and knowledge available for the underground railways, and discuss their health implications and conclusions in general.