The lines Then I built some trains And I'm travelling somewhere new It's a wonderful world, but Still no you It's a wonderful world for two expresses our believes that if we have wealth, career, love, fame, or all those other things, we can be genuinely happy, but that isn't true. we don't need all of that. Genuine happiness is as simple as being happy, but we struggle to find that, like how we don't understand simple words like "la de da de yo"
i think it's really easy to understand if it's together with the melody, and, with the context of the previous part of the song