A traditional form of public transport in Lisbon is the tram. Introduced in 1901, electric trams were originally imported from the US, and called the ''americanos''. The earliest trams can still be seen in the Museu da Carris (the Public Transport Museum). Other than on the modern Line 15, the Lisbon tramway system still employs small (four-wheel) vehicles of a design dating from the early twentieth century. These distinctive yellow trams are one of the tourist icons of modern Lisbon, and their size is well suited to the steep hills and narrow streets of the central city.