A young woman, several months pregnant, boarded a bus and sat opposite a young man, he smiled, and feeling embarrassed she changed her seat. But it was to no avail, for the young man smiled even more broadly when she sat down. Again she moved to another seat, he grinned and again after the fourth move, the young man just rolled up and roared with laughter. The woman complained and duly summoned him.

Judge: Well, young man, have you anything to say in your defense against this charge?

The young man: Well, your Honor, when the young lady entered the bus, her condition was obvious. However, that did not prompt my smile, but she sat under an advertisement that read: "Coming shortly - The Gold Rush Twins." The lady seemed indignant when I smiled and she got up and took another seat beneath a shaving stick advertisement, which read: "William's stick did the trick." She moved a third time and sat beneath a poster that read: "Sloane's liniments will remove swelling." It was after she had moved her seat fourth time that I lost control of my merriment for the above was a slogan: "Dunlop Rubber Goods would have prevented this accident."

I must be getting old, the punch line being?

@sigh@

I think there's 4 punch-lines there!