diesel engines. 88% of the lines (1.209 km) are single-track and only are double-track serving the two main routes, Messina-Palermo (Tyrrhenian) and Messina-Catania-Syracuse (Ionian), which are the main lines of this region. Of the narrow-gauge railways the Ferrovia Circumetnea is the only one that still operates, going round Mount Etna. From the major cities of Sicily, there are services to Naples, Rome and Milan; this is achieved by the trains being loaded onto ferries which cross the Strait. In Catania there is an underground railway service (metropolitana di Catania); in Palermo the national railway operator Trenitalia operates a commuter rail (Palermo metropolitan railway service), the Sicilian Capital is also served by 4 AMAT (Comunal Public Transport Operator) tramlines; Messina is served by a tramline. Airports Mainland Sicily has several airports that serve numerous Italian and European destinations and some extra-European. Catania-Fontanarossa Airport, located on the east coast, is the busiest on the island (and one of the busiest in all of Italy). Palermo International Airport, which is also a substantially large airport with many national and international flights. Trapani-Birgi Airport, a military-civil joint-use airport (third for traffic on the island). Recently the airport has seen an increase in traffic thanks to a low-cost carrier. Comiso-Ragusa Airport, has recently been refurbished and re-converted from military use to a civil airport. It was opened to commercial traffic and general aviation on 30 May 2013. Palermo-Boccadifalco Airport is the old airport of Palermo and is currently used for general aviation and as a base for the Guardia di Finanza and police helicopters. NAS Sigonella Airport, it is an Italian Air Force and US Navy installation. Lampedusa Airport. Pantelleria Airport. Ports By sea, Sicily is served by several ferry routes and cargo ports, and in all major cities, cruise ships dock on a regular basis. Mainland Italy: Ports connecting to the mainland are Messina (route to Villa San Giovanni and Salerno), the busiest passenger port in Italy, Palermo (routes to Genoa, Civitavecchia and Naples) and Catania (route to Naples). Sicily's small surrounding islands: The port of Milazzo serves the Aeolian Islands, the ports of Trapani and Marsala the Aegadian Islands and the port of Porto Empedocle the Pelagie Islands. From Palermo there is a service to the island of Ustica and to Sardinia. International connections: From Palermo and Trapani there are weekly services to Tunisia and there is also a daily service between Malta