The night train is late, the station clock ticks loudly and the waiting room is empty except for pixel Albert Einstein and a box of travel cards that has tipped over across the departures board. Every card has a twin somewhere on the board, and the stationmaster has promised that the train will not leave until each one is paired. The porters have a name for this puzzle: the Thought Express.
A thought, pixel Einstein likes to say, is like a train: once it gets going, every carriage pulls the next. Find pairs one after another without a slip and the points climb like a train picking up speed, but one wrong card stops it at the next station. Each cleared board brings a longer one, and when the clock runs out the doors close. How far can this train of thought travel?
