Been thinking about Mtambo recently and his worldwide travels before coming to Chuggington as mentioned in Season 1:
Mtambo in his early days would travel around the world to many places - from Zimbabwe to Antarctica.[1] During his travels on safari, he once came nose to nose with a rhinoceros that began to charge, but Mtambo honked his horn so loud it ran away. Deep in the jungle, Mtambo raced a jaguar, and it ran out of breath before he did. There was also a time he was lost in the desert, and along came a camel that guided him out.[2] - Chuggington Wiki
I realized I could get a few flashback episodes out of this for my fanseries. Mtambo would tell these stories to Astra, who is of course new in Chuggington and hasn't heard these yet.
The same could work for Piper, as in Season 6 we see her become a more common chugger at the safari park and even helps Mtambo from time to time. (Ex: S6E13: Piperactive, S6E43: The Record Breakers)
And if you read my headcanon for Rosa, this is build-up to her becoming Mtambo's apprentice at the Safari Park when she moves up in training.