This is even more of a blast from the past (1966!) than The Simpsons’ soapbox episode.
Bewitched (the show about a witch who twitched her nose and made stuff happen) had an episode that featured a soapbox derby. I can’t comment much on it since I didn’t watch this show as a kid (hey – we’re a youngish team, okay?), but thanks to YouTube, the entire episode is online: