Just a heads-up to anyone planning a visit in July or August. The Stadtbahn is being closed for improvement works from early July to late August, in two stages. First between Zoologischer Garten and Friedrichstraße, then from Friedrichstraße to Ostbahnhof. This will affect lines S5, S7 and S75.
It's not clear if Regional trains will be running but what I've seen suggests they are not affected.