Currently the north-south S-Bahn tunnel between Gesundbrunnen and both Yorckstrasse stations (S1, S2, S25) is closed – and will stay closed until 1:30am on 4 May 2015 (Monday).
One of Berlin’s S-Bahn official recommendations is to use ‘Ringbahn’ instead – which might be a good idea if you really want to travel from Gesundbrunnen to Yorckstrasse – or of course the other direction. Alternatively you may jump on one of the “Schienenersatzverkehr” (SEV – S-Bahn replacement shuttle buses) Buses serving the closed S-Bahn stations – more or less.
The SEV-Buses go quite frequently – every 3–4–5 minutes. There is a north branch and a south branch, meeting at S-Bahn station Friedrichstrasse.
One curiosity: the bus drivers neither sell tickets nor check tickets – just as S-Bahn drivers would.