Originally Posted by
wolf72
Noted. So, we always must get the ticket stamped before boarding the train at any time.
True for the S-Bahn and the U-Bahn, different for busses and tramways. There simply is not chance to stamp the ticket in the S-Bahn and the U-Bahn.
P.S.: And this is true for other cities with real Subway system (which Cologne does not have) and all S-Bahn trains nationwide.