Hennigsdorf factory began manufacturing the trains. The production models had lower side windows and a changed front, but were technically the same. 114 cars were built until 1982. There were 24 more, but those were delivered to Greece for a railway line there. They were returned to Berlin in 1984/85. The GII trains were returned to Athens in 1997. In 1988 a new batch of GI-trains was delivered, but with technical changes that made coupling them with the older cars impossible. Because of these changes the new trains were called GI/1. Their popular nickname was Gisela. A speciality of these cars was the fact that they had only two doors per side, unlike the other Kleinprofil trains, which had three. All the older G stock were sold to Pyongyang (GI), the G cars went for scrap, and all the GI/1 trains were refurbished into GI/1E from 2005 to 2007 in order to extend the lifespan. HK type cars In allusion to the Großprofil series H two prototypes were built in 2000, which had the designation HK - originally, the plan had been to call them A4. Unlike their Großprofil model, cars on these trains are not fully inter-connected for passengers. A full train can be divided into two half trains. The production of the first train started in May 2005. On the U2 these new trains now represent a majority of trains run. They replaced all the A3L67 trains in 2007. Majority of the trains run on the U2, while some of them ran on the U1 if there is an emergency situation like trackworks or pullover and hardly any ran on the U3 and U4. IK type cars On August 2011, BVG announced an order of IK series in order to replace A3L71 stock which will be life expired. Two prototypes have been bought from Stadler Rail and the remaining 38 units to be delivered between 2015 and 2017. These trains are based on the Stadler Tango family of trams and light rail vehicles, but will resemble and function like a full-fledged subway train. The number '1xxx' will be jumped into '18Yxxx' (where Y is the train car number). Thanks to regenerative braking the trains will recuperate up to 20% of the energy they require. Two prototype 4-car sets were initially ordered. In 2015, a production order for 38 sets was placed which are scheduled to be delivered in