Compagnia Pisana Trasporti (CPT) runs bus services both in the city and in the surrounding province. Tickets can be bought from the driver upon boarding and go up in increments according to distance. Tickets are validated at the beginning of the journey using a machine on the bus. There is also a City Sightseeing bus aimed at tourists: travellers can purchase a single ticket that is valid for 24 hours on two bus routes, one of which includes stops at the airport and the main railway station. Trains are mainly used for travelling between cities. Taxis are available in Pisa and can be called in advance or found at taxi ranks near popular tourist attractions. The Piazza Arcivescovado, where the famous tower is located, is the natural touristic travel hub of the city. Given Pisa's compact size, bicycles are another popular way of getting around, with rental shops all over the city. Otherwise, simply walking is often the best option.