When a group of four or more teenagers form a line that persistantly walks up and down the centre isle in a passenger train-obstensibly to look for seats, however, they would only consider having an totally empty set of four seats for themselves, any other permutation is rejected, hence the continuous walking up and down. Lads will do it to see what girls are aboard, ditto girls for lads. They will often swear loudly and needlessly, especially if they are looking for confrontation, as they will immediately attack anyone -verbally usually- who objects. The whole scenario is therefore often done to threaten and intimidate other passengers and is an exercise in amusement as much a practical one.
"Lets isle attack this train, there are plenty of nerds on board to worry"