You thus may be tempted to skip buying a ticket, but Germany’s “honor system” for public transport operates on the “trust but verify” principle. You never know when plain-clothes controllers will suddenly flash their badges and say the dreaded words: “Fahrkarten bitte!” (“Tickets please!”) If you get caught without a valid (stamped) ticket or pass, you’ll have to pay a fine on the spot – tourists included!There are two devices to become familiar with, the ticket vending machine (these are not at all stations, and many require correct change) and the validating machine, which might be on the platform or on the car.
It is important to know that just having a ticket in your possession isn’t enough. Your ticket must be validated, either before you board the train (using machines at the station entrance or on the platform), or immediately after you board a bus or tram (using machines in the aisle). The “Entwerter” stamps your ticket with a code for the date and time. A ticket without a stamp from the Entwerter is not a valid ticket.The time stamp establishes the "good until" condition of carriage, which rules out the presentation of a blank ticket you purchased yesterday as valid transportation today. Works the same way as the time limit on a paper transfer (for those transit operators that still issue transfers.)
Practices in Europe vary, but in Germany you can usually buy a ticket from the bus driver when you board (cash only, exact change), or using the ticket machine on trams. (In some Swiss cities you must have a ticket before you board a bus or tram.) If you already have a valid ticket, show it to the bus driver when you get on. Buses have front and rear doors. You always board in the front (“Einstieg”) and exit in the back (“Ausstieg”). Trams often have two or more cars, and you can board any one of them.