and the worn-out interiors (no DT3-LZB received the 2008 redesign), they were scrapped together with several refurbished units in 2017. 10 units are being rebuilt once again at FWM in Hennigsdorf to serve as an operational reserve for about 10 more years. They receive small propulsion modifications, the interior redesign of 2008 (most of those units were not refurbished back then), improved departure signals and structural issues of the wearing car bodies are being fixed. The newly rebuilt cars are called "DT3N", in order to distinguish them from the regular DT3E cars as the technical modifications render electric coupling between both types impossible when running in revenue service. Type DT4 The Type DT4 was introduced in 1988 to replace the Type DT1, most of the DT2 trains and some DT3 trains. The Type DT4 came with a more futuristic design, reduced noise, increased safety and greater economy than the older trains. A total of 126 units were built in six batches from 1988 to 2005. Due to the long construction span the batches slightly differ from each other in terms of interior and exterior design, propulsion and destination signs. Also the first two batches had doors operated by handles, which were replaced by buttons around 2004. Since 2011 the older DT4 trains are being refurbished as the oldest units are almost 30 years old already, by July 2018 the first 50 of 126 units have been refurbished. Additionally the electrical equipment (including propulsion) of the units 141 and 143–150 has been replaced by the equipment used in the sixth and youngest batch in order to receive spare parts for the aging first two batches, making mixed usage of these units with any other batch than the sixth impossible as their propulsion is incompatible to the one used in older units. Type DT5 The Type DT5 was introduced in 2012 to completely renew the current rolling stock. By July 2018, 89 of 118 planned units have been built and delivered. The Type DT5 is the first type of U-Bahn trains in Hamburg with air-condition and transitions between the cars. More DT5 units were ordered than necessary for replacement of all DT2E and DT3E units, in order to enlarge the total number of cars in revenue service to be able to provide additional service in the future. Former rolling stock Type T/TU The HHA Type T was introduced with the Hamburg U-Bahn