 The first generation (series 2000) is currently being retired, while the second generation (series 2100) is due to receive a major overhaul for at least another 15 years of service.After tests with a low-floor tram from Vienna, a similar vehicle was developed for Cologne by Bombardier Transportation, later becoming part of Bombardier's standardized Flexity Swift family.A total of 124 low-floor trams were delivered between 1995 and 1998, referred to as "K4000".