since 1984, is the oldest underground mass transit system in India. It spans the north–south length of the city. In 2020, part of the Second line was inaugurated to cover part of Salt Lake. This east–west line will connect Salt Lake with Howrah. The two lines cover a distance of . , four Metro rail lines were under construction. Kolkata has five long-distance railway stations, located at Howrah (the largest railway complex in India), Sealdah, Kolkata, Shalimar and Santragachi, which connect Kolkata by rail to most cities in West Bengal and to other major cities in India. The city serves as the headquarters of three railway zones out of eighteen of the Indian Railways regional divisions—the Kolkata Metro Railways, Eastern Railway and the South-Eastern Railway. Kolkata has rail and road connectivity with Dhaka, the capital of Bangladesh. Buses, which are the most commonly used mode of transport, are run by government agencies and private operators. Kolkata is the only Indian city with a tram network, which was operated by the Calcutta Tramways Company. It has now renamed to West Bengal Transport Corporation. The slow-moving tram services are restricted to certain areas of the city. Water-logging, caused by heavy rains during the summer monsoon, sometimes interrupt transportation networks. Hired public conveyances include auto rickshaws, which often ply specific routes, and yellow metered taxis. Almost all of Kolkata's taxis are antiquated Hindustan Ambassadors by make; newer air-conditioned radio taxis are in service as well. In parts of the city, cycle rickshaws and hand-pulled rickshaws are patronised by the public for short trips. Due to its diverse and abundant public transportation, privately owned vehicles are not as common in Kolkata as in other major Indian cities. The city has witnessed a steady increase in the number of registered vehicles; 2002 data showed an increase of 44% over a period of seven years. , after adjusting for population density, the city's "road space" was only 6% compared to 23% in Delhi and 17% in Mumbai. The Kolkata Metro has somewhat eased traffic congestion, as has the addition of new roads and flyovers. Agencies operating long-distance bus services include the West Bengal Transport Corporation and various private operators. The city's main bus terminals are located at Esplanade and Babughat. The Kolkata–Delhi and Kolkata–Chennai prongs of the Golden Quadrilateral, and National Highway 12 start from the city. Netaji Subhas Chandra Bose International Airport, located in Dum