toronto's main public transportation system is operated by the toronto transit commission (ttc). the main part of its public transport network is the toronto subway system. that system includes three heavy-rail rapid transit lines spanning the city, including the u-shaped line 1 and east–west line 2. a light metro line also exists, exclusively serving the eastern district of scarborough. a discussion is underway to replace it with a heavy-rail line. the ttc also operates an extensive network of buses and streetcars. the streetcars serve the downtown core. the buses provide service to many parts of the city not served by the sparse subway network.