What can make a train full of stoic Germans sing and dance?

Answer: The Beatles.
Now let me give you the back story. Take a ride on any Berlin S-Bahn or U-Bahn and you'll immediately notice that it is not a very social experience. People sit silently, listen to music, read books and don't make eye contact. When people do talk it's normally in quiet whispers. Often young musicians will get on a play their accordion or violin for a few minutes in hopes of making a few Euros but mostly they are ignored (for good reason, too, because a lot really aren't that talented).
Therefore, when three young guys whipped out guitars as I was riding the train home last night I barely gave them a glance before going back to my book. But then something remarkable happened. They started playing the opening bars to The Beatles "All My Loving" and the energy in the train car completely changed. People perked up, turned around, smiled and started tapping their toes along to the music. The stoney-faced man sitting next to be broke into a wide grin and started tapping out the beat on his leg. A woman across started dancing in her seat and the whole train car of fifty or so people all simultaneously broke into song. .
I absolutely couldn't believe my eyes, or my ears, but a whole car on the 10:35 train towards Potsdam were smiling, dancing, laughing and singing together. Those five minutes in the train are one of my favorite experiences in Berlin thus far. It was one of those moments when you feel completely familiar with a group of complete strangers, all connected by the simple enjoyment of a song. How fortuitous and serendipitous that I happened to step onto that car of that very train. Once the song was over, we all paid our fifty cents of gratitude and then returned to our silent journey but I think that we all went home a little happier, quietly humming The Beatles to ourselves.