Use the largest animal to set the size of your train cars. Take an A4 or Letter size cardstock and fold in half. Place the animal inside the folded sheet. Trim the car's height so that it reaches just above the animal's legs and the width should be roomy enough for the animal to slip in and out of the car.
Use the train to learn more about animals. Assign one animal per car and let kids draw, cut out, or write things they learn about the animal. Slip these inside the animal's train car. You can install additional cars as kids learn about more animals. Here are some useful links to animal learning for kids: