This is a fantastic fantasy for highlighting the destruction we humans have done to the Earth, specifically regarding animal habitat encroachment and destruction.
However, my daughter pointed out it's ironic the author chose a coal-burning train, specifically, as the "vehicle" of enchantment and the animals' salvation, as the burning of fossil fuels is directly responsible for global warming and rapid loss of habitat of polar ice-cap animals like the polar bear featured in the text. Uncle Herbert, at least, drives a tricked-out electric Tesla.
Kate gets a present from her Uncle Herbert: a train. A real train, engine and all. It's not the kind of thing a kid typically gets for their birthday, but when she sees the fire burning in the engine that night, she and her brother, Tom, head out to investigate and before you know it, they're on a world-wide journey, taking animals' tickets and transporting them to places that aren't so crowded with humans they can no longer live.
They stop a few "invading species," like a boar and some starlings, who don't have tickets, from boarding and rescue a baby Pangolin and a critically endangered white-bellied, or Imperial, crane, and a polar bear, whose iceberg station has completely melted and is exhausted and almost drowns from swimming so long while waiting for the train.
At one point they pass her Uncle's train, one he abandoned, metaphorically representing the older generation's failed attempts to stop animal habitat destruction and climate change.
It's a quick read, if you don't think too closely about the vehicle for the message.
There is also a free Teaching Notes guide, complete with four (4) excerpts with comprehension questions, a graphic organizer for human impacts on the environment, and a project to plan an animal habitat rescue train ride.