▁Kar ls ru he ▁Ost stadt . ▁ ▁On ▁ 3 ▁March ▁ 2 0 1 1 , ▁construction ▁started ▁on ▁a ▁line ▁from ▁T ull ast ra ße ▁via ▁Sch lach th of straße ▁and ▁Ludwig - Er hard - Al lee ▁to ▁Ba ume ister straße . ▁The ▁ 2 . 2   km ▁long ▁route ▁with ▁four ▁new ▁stops ▁was ▁opened ▁after ▁ 1 8 ▁months ▁of ▁construction ▁on ▁ 8 ▁September ▁ 2 0 1 2 ▁at ▁a ▁cost ▁about ▁€ 3 0 ▁million . ▁It ▁is ▁served ▁by ▁an ▁extension ▁of ▁tram ▁line ▁ 6 ▁from ▁Kar ls ru he ▁Haupt bahn hof ▁at ▁ten - min ute ▁intervals . ▁ ▁Stadt bahn ▁tunnel ▁ ▁Since ▁the ▁ 1 9 6 0 s , ▁there ▁have ▁been ▁attempts ▁to ▁build ▁a ▁tram way ▁tunnel ▁in ▁the ▁centre ▁of ▁Kar ls ru he ▁to ▁rel ieve ▁the ▁main ▁sho pping ▁street ▁of ▁tr ams . ▁However , ▁these ▁plans ▁always ▁failed ▁because ▁of ▁the ▁cost ▁of ▁such ▁a ▁project . ▁As ▁the ▁volume ▁of ▁tr ams ▁on ▁the ▁central ▁cor rid or ▁along ▁Kaiser straße ▁continued ▁to ▁grow ▁with ▁the ▁expansion ▁of ▁the ▁tram way ▁and ▁the ▁Kar ls ru he ▁Stadt bahn ▁network ▁in ▁the ▁ 1 9 8 0 s ▁and ▁ 1 9 9 0 s , ▁further ▁plans ▁for ▁a ▁tunnel ▁route ▁were ▁made ▁to ▁rel ieve ▁the ▁surface ▁tram ▁line . ▁After ▁an ▁initial ▁re jection ▁at ▁a ▁refer endum ▁in ▁ 1 9