OK, assuming that he got on/off the train without riding, laving a
parcel there, and not raising any suspiscion and nobody noticed it that
got off at a later stop and mentioned it to the police/media...

The bombs were timed, they exploded simultaneously. That means whoever
planted the piccadilly line one left the train seconds before the bomb
went off. That's cutting it *very* close. What if the train has been a
couple of minutes later - the bomb would have gone off in the guys
hand.