Today there are two different railway operators: Trenitalia which connects the most populated towns and the main ports. This network is the most modern on the island, running primarily diesel locomotives such as the Alstom Minuetto and, from 2015 the faster tilting train CAF ATR365 and ATR 465, specifically designed for the Sardinian railway network; ARST: the trains run on narrow-gauge track, are generally slow, due to the tortuosity of the lines, except for the electrified tram-trains operating in the metropolitan areas of Sassari and Cagliari. The Trenino Verde (Little Green Train) is a railway tourism service operated by ARST. Vintage railcars and steam locomotives run through the wildest parts of the island. They allow the traveller to have scenic views impossible to see from the main roads. Demographics With a Population density of 69/km2, slightly more than a third of the national average, Sardinia is the fourth least populated region in Italy. In the recent past the population distribution was anomalous compared to that of other Italian regions lying on the sea. In fact, contrary to the general trend, most urban settlement, with the exception of the fortified cities of Cagliari, Alghero, Castelsardo and few others, has not taken place primarily along the coast but in the subcoastal areas and towards the centre of the island. Historical reasons for this include the repeated Saracen raids during the Middle Ages and then Barbary raids until the early 19th century (making the coast unsafe), widespread pastoral activities inland, and the swampy nature of the coastal plains (reclaimed definitively only in the 20th century). The situation has been reversed with the expansion of seaside tourism; today all Sardinia's major urban centres are located near the coasts, while the island's interior is very sparsely populated. It is the region with the lowest total fertility rate (1.087 births per woman) and the second-lowest birth rate of Italy (which is already one of the lowest in the world). Combined with the aging of population going rather fast (in 2009, people older than 65 were 18.7%), rural depopulation is quite a big issue: between 1991 and 2001, 71.4% of Sardinian villages have lost population (32 more than 20% and 115 between 10% and 20%), with over 30 of them being at risk to become ghost towns. It is predicted that, at this rate, Sardinia is going to be the European island with the lowest population density