This is truly a shot in the dark.
I vividly remember a childhood friend in the late 90s/early 00s having a Kelly (?) circus set. In my memory it had a circus train with three cars that were open air pink plastic with working wheels, and an animal. It was too small to have been for Barbie and had a smaller doll with it who I vaguely remember as being blond in a circus-performer outfit.
My friend did have an older sister so it may have been from an earlier era.
I have scoured the internet and cannot find any circus or carnival Barbie that matches my memory. Do you (or your followers) have any idea if this was a real toy I always wanted to play with or something I’ve made up?
You sent this a while ago and I didn't reply at the time because I was experiencing a chronic bout of can't be motivated -- but I have been thinking about this on and off since I first saw it and thought I'd put a bit of thought into this one.
There have been some Kelly Club circus-adjacent sets, like Kelly and her friends in various circus/carnival outfits.
But no circus themed playset that I am aware of.
I do want to draw attention to this Nursery School playset from 1997, though.
Apart from the fact that it isn't circus themed, this fits your description fairly well. Pink train with open air carriages, sized for a Kelly doll, from the late 1990s to early 2000s. No animal, and no blonde circus performer though.
But is it possible that this was the train, and the animal and circus performer doll were from different releases and you and your friend just put them together to turn this nursery school train into a circus train?
It's also possible that whatever this was wasn't a Barbie release -- there's a lot of other fashion dolls and kid dolls on the market in other brands that sometimes confuse our memories. If it turns out this train isn't what you remember, that's my best guess.