I'd echo ChiefPlanner and recommend a trip round the Ring, which will let you see not only a huge amount of stuff of current and historic railway interest but also a great deal of urban and social interest about this fascinating city itself. And the Technology Museum is really worth a visit. If you want to get an impression of the scale of the railway works that have been undertaken in recent years, get off the S-Bahn at Westhafen, walk around a bit, and see the civil engineering involved in joining the new tunnel main lines and S-Bahn lines to the Ring in both directions. If you have the time, take a trip around the Außenring and see what the East German state did to avoid its trains having to come into West Berlin. There really is so much to see -- and that's before you even start on the marvellous museums and galleries or take a trip on the waterways. Oh, and do take a look at Goering's Luftfahrtministerium which survived the war pretty well and has been used by every government since to house various departments of state, and also at the massively ostentatious Russian Embassy, built very soon after the war.