I'm riding the central line at six pm on a friday. A guy in front of me with a faux anarchist haircut looks nervous. He checks his watch, his phone, his map, his nails, his watch, his phone, his map... He bounces on the balls of his feet. He is carrying a slightly oversized and narrow violin case (an anarchist violinist? Is he warping Vanessa Mae?). There is a sticker on the case. It says "bam".

Suddenly I'm nervous.

Is tonight when the Christmas lights are being turned on by the stars of a Christmas Carol? The celebration of the most obvious street crossing system of all time? Have I wandered into a prime terrorist opportunity?