5-minute frequency during rush-hour. The construction of underground railways in the last decade has resulted in the trams losing some of their importance to the city's transport infrastructure. Despite this, two new sections have been constructed in recent years. It is now appreciated that trams are more attractive to the travelling public than buses and cheaper to build than underground railways. Therefore, neighbourhoods with medium transit usage are either currently served by tram routes or will be in future. Underground There are 9 Stadtbahn lines known as the U-Bahn. The U1–U3 run from the southern railway station to the north of the city on a common route. They then split and serve Nordweststadt (U1) the town of Bad Homburg-Gonzenheim (U2) and the town of Oberursel (U3) The U4 runs from Bockenheimer Warte, through the fair, the main train station, the city centre to Bornheim (Frankfurt am Main) Seckbacher Landstraße. Since June 2008 every 2nd or 3rd train continues to Schäfflestraße with connection to the U7. This route has been the only line which runs entirely underground, except from the part behind Seckbacher Landstraße. The U5 is a combined tram and underground line heading from Preungesheim to the city centre. There it co-shares four underground stations with the U4. The U6 runs from Heerstraße in the west, to Ostbahnhof (East Station) in the east. The U7 runs from Hausen in the west, to Bergen-Enkheim in the north-east. The U8 and U9 serve a new connecting section between the U2 and U3 northern ends. U8 goes from Riedberg in the new section of track to Frankfurt-Süd sharing the tracks with U1-3. U9 starts from the north at Nieder-Eschbach on the (shared) U2 line and goes along the new track that it shares with U8, then along a short section of U3, and finally connecting to the U1 tracks to Ginnheim in Nordweststadt. In city centre sections the rail tracks are underground, but run on their own separated section at ground level in the suburbs. The minimum service interval is 2.5 minutes, although the usual pattern is that each line runs with a 7.5- to 10-minute frequency which combines to approx 3–5 minutes on the city centre sections served by more than one line. The trains can be as long as 300 ft (100 m) and have a maximum speed of approximately 50 miles per hour (80 km/h). Cars are equipped with indicators