The Ponte Luís I, often called the Ponte Dom Luís, connects the central parts of Porto and VN de Gaia. It had and has a major role in the public transport of Porto and is one of the most important landmarks of the city. This bridge with two decks was built in the 1880’s as successor of the from 1843 dating Ponte Pênsil.
Line D is the only line not using the joint route between Estádio do Dragão and Senhora da Hora. It has a 4 km long tunnel between Polo Universitário and the bridge across the Douro river with 8 stations. For the river crossing the upper deck of the complete renovated Ponte Luís I is used. The upper deck of the bridge and the adjacent Avenida da República in VN de Gaia had electric trams from 1905 until 1959.