It's true about the Sun. It's the most popular paper in England, yet hardly anyone on the Brighton line reads it. In fact, I'd say about three times as many people read the Guardian than read the Sun.
Although, sadly, most people seem to read Metro.
Conclusive proof, as if any were needed that:
1) Commuters are the economic blood in the veins of this great nation. The Sun is beneath these folk (although if someone in the office has a copy, they'll pick it up for a few minutes although just for the sport you understand).
2) Commuters are in it for the money. The Metro is cheap. Very cheap. By getting the Metro instead of, say, The Times, a commuter saves 70p. Multiply that up over a year and even taking account of holidays, that's a good £150 saved. (And, besides, one can generally be picked up for free, for the journey home, once the train arrives in London from all those mugs who spend all that cash and then just discard their purchase.)