[[File:Schienennetzplan Frankfurt am Main.png|thumb|S-Bahn, U-Bahn and tram lines (schematic map) Additional information: between the stations Stadion and Flughafen Regionalbahnhof a further station exists, name: Frankfurt am Main gateway gardens']] The public transit system in Frankfurt is part of the Rhein-Main-Verkehrsverbund (abbreviated: RMV) transport network and consists of several carriers who all use the same fare system. Therefore, one ticket is valid for a journey which may include several modes of transit run by different operators. The fares are paid in advance of travel at a ticket vending machine or at the driver on board a bus. There are no turnstiles or other controlling barriers; instead, a proof-of-payment system is used. Plainclothes fare inspectors are employed and carry out random checks to ensure passengers have paid. If found to be travelling without a ticket, then they are required to pay a fine of €60. A single way trip within Frankfurt costs between €1.80 and €2.80, and a journey to the suburbs outside Frankfurt costs between €4.65 and €9.10 approx. There are also some discounts for children for groups or day tickets. A day ticket for traveling within Frankfurt costs €7. Rapid Transit Frankfurt itself is the central city of the so-called Rhein-Main Region, and therefore the service caters the large number of commuters who travel to and from the suburbs each day. The suburban rail service is provided by rapid transit trains (S-Bahn). All but one of the lines run together in a single underground section through the city centre, and then go their separate ways at either end. In the suburban sections they are generally double-tracked or share mainline tracks with other regional, long distance passenger and cargo trains. The trains can be up to 200 m (600 ft) long and have a maximum speed of approximately 140 km/h (90 mph). Electricity supply is provided by overhead wires at the common German railroad voltage (15,000 volts AC). The normal service interval on each of the lines is 15 or 30 min, but on the central sections a train runs approximately every 2 minutes during rush hour and the S8/S9 runs 24/7. Frankfurt's rapid transit is very popular because it is fast. It has the added benefit of being very cost-effective for medium-distance lines with high usage. Trams There are 9 Tram lines, with trams arriving usually every 10 min. Many sections are served by two lines, combining to give a