To the sound of waves and seagulls, a man and woman walk together with their arms around each other on a seashore boardwalk.
A man walking the other way suddenly catches the eye of the other man, who suddenly abandons the woman without a word or care and walks the other way with the fellow, holding hands with him. The woman is stunned and says nothing.
Narration says, "Over 96% of our trains get their connection trains. German State Railways giving you connections."
The implication is easy connections between switching/connecting of trains and sexualities. While the ad could be read as referring to bisexuality, the joke seems to be that straight people can simply switch to gay -- an inaccuracy -- if someone attractive comes along, and completely disregards the previous relationship.