There Are No Turnstiles or Gates – Berlin's U-Bahn is based on an honor system. The ticket machines are on the platforms, but there seems to be nothing stopping you from boarding the train without your ticket. However, police patrol the subways constantly, sometimes disguised as civilians and randomly check passengers for their tickets. If you do not have a valid ticket or lost your ticket, then you have to pay a very high fine. If this happens the 3rd time, you have to go to court. Do not risk this, and always travel on the U-Bahn with a valid ticket.
You Have to Manually Open the Subway Doors – This concept is awkward for foreigners who are used to the subway cars opening automatically. On the Berlin U-Bahn, you have to either pull a latch or press a button to open the train door, either from the inside or the outside. If someone presses it first, then you are free to enter or exit.
The Train Stations are Very Long – Berlin is a very large city and the trains are very long. Therefore there is an entrance to the U-Bahn at one end of the subway platform and at the other end. These entrances are a long distance apart, so if you get out at the wrong exit, you may have to walk far to get where you really wanted to go.
The Ticket Machines are Finicky With Cash – The ticket machines in the U-Bahn do not like making change, so paying for your ticket in exact change or close to exact change is preferred. It also only accepts very crisp new Euro bills. You can alternatively pay with a debit card, but not a credit card where you would have to sign.
The Trains Run Very Frequently – If you miss a train as you enter the station, relax, because the next one will come within 2-5 minutes. You will typically wait no more than 10 minutes for a train, but most come every few minutes, and are rarely delayed. Since the U-Bahn system is largely dependable, you can time your commute fairly well once you travelled it once.