There is a dense network of bus routes, run by the municipal company Empresa Municipal de Transportes (or EMT Madrid), which operates 24 hours a day; special services called "N lines" are run during nighttime. The special Airport Express Shuttle line connecting the airport with the city centre features distinctively yellow buses. In addition to the urban lines operated by the EMT, the green buses (''interurbanos'') connect the city with the suburbs. The later lines, while also regulated by the CRTM, are often run by private operators.