reconstruction of all north-south lines (S1, S2, S6) was to commence in 2011. The investigation committee of the Senate of Berlin pointed out that the admission certificate of the most common traction vehicles will run out in 2017 - given the (non-fatal) train accidents in 2001 and 2002 due to exceeding speed (that allowed to overrun the overlap distance hitting another train that was still in the station) it was expected that the Eisenbahn-Bundesamt regulator will require a modern train stop system to be installed during the renovation as a prerequisite for a renewed admission. Prolonging the old admission was expected to be unlikely to be done for more than three years making the ZBS train control system a requirement for any movement authority in the S-Bahn rail network presumably by 2018 and no later than 2020. The allowance for the continued usage of the old track stops was originally limited to 2014 as it was expected that the migration to the new system would be complete until then. With a motion to the Federal Railway Authority the migration phase to ZBS was extended to 2025. The long new time frame come from delays in the procurement of a new operator for the Berlin Ringbahn tracks - the new operator may continue to run the old train types BR 480 and BR 485 (without ZBS) until 2023 when then new operator is required to have introduced an all new series of trains (with ZBS). System Although the system is based on ETCS Level 1 it is not compatible. Most notably, the system has dropped the usage of Euroloop line signalling - instead additional infill balises are placed between the distant signal and the stop signal. A semaphore on the distant signal flags to use a lower speed in approaching the stop signal - in most situations in rapid transit a stop signal will be dropped before reaching the stop semaphore. To increase throughput on the main lines the infill balises can send an upgrade message to the cab signalling electronics. If the train was required to halt then the train driver is responsible for interpreting the movement authority of the semaphore signal and to move the train slowly up to the next balise past the signal mast so that the cab control computer can see the release. Using a special design allows the reuse of the old wayside signals -