By far the most obscure joke in the book, this thing with a kid in the hedge with a stopwatch and a ledger happens 5 or 6 times throughout. See, Trainspotting (Irvine Welsh's book) took its name from this bizarre fad that thrived briefly in Great Britain. Apparently, people would take up positions by the train tracks and make sure they ran on time, as a service to British Railway for which they received money. People would keep logs, compete for the best positions, fight over territory, and so on. Madness.

 The joke of students "trainaspotting" while Traina wandered around was obviously not going to be understood by everyone. However, I don't mind that, because even though 1 in a 1000 will get it, that one person will find it a million times funnier than his friend to whom he will have to explain the joke.