(including one prototype), and six have been scrapped. As with A trains, up to three B double-carriage units can form a 3/3 train (Langzug). However, it is not possible to form a mixed train of A and B units, which are not compatible. Class C Class C trains were designed in the late 1990s to gradually replace the Class A trains, which become more expensive to operate and maintain. Class C1 trains are six carriages attached together in one continuous length with gangways between the carriages, allowing the passengers to walk from one end to other. Ten trains, designated as C1.9 (Wagon No. 601–610), were ordered and delivered without prototype trains for evaluation. The technical difficulties delayed the entry into service to 2002. Eight more units, designated as C1.10 (Wagon No. 611–618), were delivered in 2005 prior to the 2006 FIFA World Cup. The total number of C1 trains in operation is 18. A second generation, designated as Class C2, were ordered from Siemens Mobility in November 2010. They are based on new Siemens Inspiro subway trains. The first delivery of 21 units (C2.11) began in 2012 but was delayed until 2018 due to the technical defects and changes in federal and state regulations. The second order for 22 units (C2.12) was announced in 2019 with delivery commencing in 2020 and continuing until 2022. The third order for 24 units (C2.13) was placed in May 2020 for the delivery from 2022 to 2024. The total number of C2 trains in operation and under construction is 67. Operation MVG has made lot of effort in the last few years to increase the frequency for trains on the busier lines (U1/U2/U7 and U3/U6) to five minutes between trains and to ten minutes on less busy lines (U4 and U8). The busier lines (U1/U2/U7/U8 and U3/U6) have "Langzug" (3 Class A/Class B trains or 1 Class C1/C2 trains). "Kurzzug" (short train) with 1 or 2 Class A/Class B trains are operated on less busy lines (U4/U5) or during less busy times in the evenings or weekends. During the Covid-19 pandemic in 2020, the "Kurzzug" has been temporarily removed from the service. U3/U6 lines on shared tracks within city boundary has a notorious reputation of frequent service interruptions due to unanticipated technical issues, overcrowding, and passengers requiring medical attention or falling into the tracks at the stations. Additionally, U3/U6 lines receive frequent maintenance and