the Collective

Never assume you know exactly what everyone else is thinking...
http://xkcd.com/610/
In this cartoon, irony is the primary mover and shaker. The cartoon's anonymous author obviously wanted to convey a couple of messages:
1) There is a collective conscious, a collective mindset and general trend of ideas for groups of similar people. The general idea here is that everyone is "asleep" in the sense that they don't make their own decisions, that they are all subject to influence of forces that the subjects feel they are above the influence of.

2)We are all subject to these influences, it is blindingly obvious, and we are all deluding ourselves by thinking otherwise.

The evident irony is part of the dark, bleak humor associated with many cartoons depicting society. It appears that the train car depicted is full of people who belive they're the only "real" human left, potentially lending a hand to the prominance of technology in everyday life, for almost every individual.