They have all sung the word, "train".
For my two-year-old son that is enough commonality to make a killer playlist. This is great news for me because linking songs together by a common word or theme is about the extent of my dj-ing talent.
My junior high mix tapes sucked. There was no flow, just awkward breaks from one song to the next (if you were lucky enough for the song not to be cut short before the next) and it really didn't matter if one song led into the next one smoothly...and mostly I was recording the songs off the radio. BUT this no longer matters! My son is fully satisfied with my mixing skills and so Ozzy's "Crazy Train" is unapologetically followed by "Midnight Train to Georgia" and everyone is happy.