The public transport infrastructure consists of a heavily used Prague Integrated Transport (PID, ''Pražská integrovaná doprava'') of Prague Metro (lines A, B, and C – its length is  with 61 stations in total), Prague tram system, Prague buses, commuter S-trains, funiculars, and six ferries. Prague has one of the highest rates of public transport usage in the world, with 1.2 billion passenger journeys per year. Prague has about 300 bus lines (numbers 100–960) and 34 regular tram lines (numbers 1–26 and 91–99 ). There are also three funiculars, one on Petřín Hill, one on Mrázovka Hill and a third at the Zoo in Troja.