Word Count: 4885 (text) + 5x250 (tables) + 5x250 (figures) = 7385 words ABSTRACT Residential areas consisting of detached houses were built in North and East Madrid during the last decade of the twentieth century. This implies that the population density in those areas is very low and, therefore, implementing an efficient transport system is complicated. In South Madrid in the 60s, however, huge commuter towns developed. Some of them were linked to a suburban network at some point, while others were very far away. The Madrid region promoted the construction of several light rail lines to resolve the issue. The lines function as extensions to the existing rail network (subway or suburban trains). This article describes those lines and analyses the determining factors that enabled them to be implemented and operated successfully. The recommendations for implementing a LRT System, based on that analysis, are: the lines should not be very long; they should have a segregated right-of-way; the quality of service should be good, and they should be coordinated with other modes of transport. Apart from public funding, they could be funded by additional resources collected from property tax and the concessionaire company (private funding). A concessionaire consortium may comprise construction companies, transport operators, financial institutions, rolling stock manufacturers and consultancies. Finally, the concession should be granted for a 30 to 40-year period so private stakeholders can recover their investment.