Istanbul's primary motorways are the O-1, O-2, O-3, and O-4. The O-1 forms the city's inner ring road, traversing the Bosphorus Bridge, and the O-2 is the city's outer ring road, crossing the Fatih Sultan Mehmet (Second Bosphorus) Bridge. The O-2 continues west to Edirne and the O-4 continues east to Ankara; the O-2, O-3, and O-4 are coterminous with European route E80 (the Trans European Motorway) between Portugal and the Turkish–Iranian border. The two Bosphorus Bridges currently form the only fixed links between the Asian and European sides of Turkey, together carrying 400,000 vehicles each day. The dual-deck, 14.6-kilometer (9.1 mi) Eurasia Tunnel is currently under construction beneath the Bosphorus, between Kumkapı, Fatih and Selimiye, Üsküdar. The Third Bosphorus Bridge, first considered in the 1990s, may also finally be coming to fruition, as the project was officially launched in 2012. Both projects may be completed as early as 2015. Environmentalist groups worry that the third bridge will endanger the remaining green areas to the north of Istanbul. stanbul's local public transportation system is a complex network of trams, funiculars, metro lines, buses, bus rapid transit, and ferries. Fares across modes are integrated, using the contactless Istanbulkart, introduced in 2010, or the older Akbil electronic ticket device. Trams in Istanbul date back to 1872, when they were horse-drawn, but even the first electrified trams were decommissioned in the 1960s. Operated by Istanbul Electricity, Tramway, and Tunnel (İETT), trams slowly returned to the city in the 1990s with the introduction of a nostalgic route and a faster modern tram line, which now carries 265,000 passengers each day. The Tünel opened in 1875 as the world's second-oldest subterranean rail line (after London's Metropolitan Railway). It still carries passengers between Karaköy and İstiklal Avenue along a steep 573-meter (1,880 ft) track, while a more modern funicular between Taksim Square and Kabataş began running in 2006. The Istanbul Metro comprises three disconnected lines (the M1, M2, and M4) with several other lines (including the M3) and extensions under construction or proposed. The two sides of Istanbul's metro will ultimately be connected under the Bosphorus when the Marmaray tunnel, the first rail connection of any kind between Thrace and Anatolia, is completed in 2015. Upon its completion, rail use in the city is expected to increase to 28 percent (from just 4 percent), behind only Tokyo and New York City. Until then, buses provide transportation within and between the two halves of the city, accommodating 2.2 million passenger-trips each day. The Metrobus, a form of bus rapid transit, traverses the Bosphorus Bridge, with dedicated lanes leading to its termini. İDO (Istanbul Seabuses) runs a combination of all-passenger ferries and car-and-passenger ferries to ports on both sides of the Bosphorus, as far north as the Black Sea. With additional destinations around the Sea of Marmara, İDO runs the largest municipal ferry operation in the world. The city's main cruise ship terminal is the Port of Istanbul in Karaköy, with a capacity of 10,000 passengers per hour. While most visitors enter Istanbul by air, about half a million foreign tourists enter the city by sea each year. International rail service from Istanbul launched in 1889, with a line between Bucharest and Istanbul's Sirkeci Terminal, which ultimately became famous as the eastern terminus of the Orient Express from Paris. Regular service to Bucharest and Thessaloniki continued until the early 2010s, when the former was interrupted for Marmaray construction and the latter was halted due to economic woes in Greece. After Istanbul's Haydarpaşa Terminal opened in 1908, it served as the western terminus of the Baghdad Railway and an extension of the Hejaz Railway; today, neither service is offered directly from Istanbul. Service to Ankara and other points across Turkey is normally offered by Turkish State Railways, but construction of Marmaray and the Ankara-Istanbul high-speed line forced the station to close in 2012. New stations to replace both the Haydarpaşa and Sirkeci terminals, and connect the city's disjointed railway networks, are expected to open upon completion of the Marmaray project; until then, Istanbul is left without intercity rail service. Private bus companies instead operate routes along—and well beyond—those offered by the rail network. Istanbul's main bus station is the largest in Europe, with a daily capacity of 15,000 buses and 600,000 passengers, serving destinations as far as Frankfurt. Istanbul has two international airports, the larger of which is Istanbul Atatürk. Atatürk, located 24 kilometers (15 mi) west of the city center, handled 45.1 million passengers in 2012, making it the sixth-busiest airport in Europe and the twentieth-busiest in the world. Sabiha Gökçen International, 45 kilometers (28 mi) southeast of the city center, opened in 2001 to relieve Atatürk. Dominated by low-cost carriers, Istanbul's second airport has rapidly become popular among travellers, especially since inaugurating a new international terminal in 2009; the airport handled 14.7 million passengers in 2012, a year after Airports Council International named it the world's fastest-growing airport. Atatürk has also experienced rapid growth, as its 20.6 percent rise in passenger traffic between 2011 and 2012 was the greatest among the world's top thirty airports. Because of the traffic at Istanbul's current airports, a third international airport is planned for the Black Sea coast. Building a new runway at Atatürk Airport was rejected due to the cost involved; however, environmental concerns have also been raised with respect to the plans for a third airport.