We soon learned that these metro trains signal their imminent departure by means of a red light above the door and a horn-type bleep, and that they hang around in the platform for a good few seconds longer than a typical tube train. On we hummed, initially uneventfully, and then the ticket inspectors got on. They'd looked quite ordinary waiting on the platform, one little and one large, before suddenly revealing their ID and starting to check our credentials. Refreshingly every platform in Berlin is ungated, and the populace are trusted to validate their ticket before travelling (and occasionally checked upon). On this occasion everything checked out until the guards reached the stocky middle-aged German couple, whose papers were unexpectedly found wanting, and they were escorted onto the next platform to face a fine.
Our next unusual passenger was a down and out with a dog. Again the dog was muzzled, this being one of the U-bahn's byelaws (along with no smoking, no alcohol and - by the looks of the graphic - no chips). This vagrant had brought some printed material to sell, I think Berlin's equivalent of The Big Issue, and started working his way (entirely unsuccessfully) through his potential customers. We merely shrugged, having nothing more than O-Level German with which to converse, and on he pressed. Then suddenly the three young casual lads down the end of the carriage gathered around him, said something, laid a hand on his shoulder and whipped out a sheet of papers. They were undercover law enforcement, it seems, and within seconds he too was off the train being dealt with on the platform.