Q: 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  ---This part describes our journey on finding happiness, we realize the world is beautiful, but we still can't find happiness (referenced by "still no you", you references happiness)
A: you mentioned this out of order, but this comes after the "just too small" line. Perhaps the 'built some trains', 'traveling somewhere new' was my way getaway scheme, to escape from the blandness of the previous line. Or just assuming the mall was not a viable option. it is a natural progression to take a train somewhere else.... but with 10,000 bonus points for building your own train.