In such situations you have to identify yourself somehow, that's what obligation of identification means. I was with a friend in the same situation (he forgot

In such situations you have to identify yourself somehow, that's what obligation of identification means. I was with a friend in the same situation (he forgot his ticket and had no ID). I just had to witness that he was indeed the person he said he was and it was fine. There are many situation where in the US the police will immediately arrest you while the German police will just write down your identification and let you go for now. I will rather take the German approach.