Berlin has a good public transport system that is very accessible to wheelchair users. In the west of the city is a system of bus routes that are accessible, though I didn’t use them. Then there are four sorts of rail transport three of which I used last weekend. The Berlin transport authority produces a map that shows which S-Bahn and U-Bahn stations have lifts or ramps, though the ramps such as at Schönefeld airport can be quite steep.
Regional trains are probably the first you will come across if you fly in to Berlin Schönefeld airport. These are usually red coloured trains and are good for travelling longer distances in the area as they stop at fewer stations. They have a carriage that has access for wheelchairs and bicycles. There is a button at the door, both inside and outside, to call for a ramp. At Schönefeld station the access is level so you can board unaided, it is possible to board at any door but you MUST board at the correct carriage or there will be no space for your wheelchair and you will be blocking the vestibule. And more importantly there will be no ramp to get off at your destination. Unlike any other trains I have travelled on you have to go down a ramp, rather than up, to board these trains.