I've included a small container of animals in amongst Ed's trains and cars and these are being incorporated in his play more and more. Sure, they generally ride around in the back of a ute or train carriage, or the cars and trains visit a 'zoo' he has set up, but I'm glad to see him including the animals in his made up stories.
Branching out into a new interest area like this helps development in so many ways. It builds on his mental catalogue of objects in his world, builds his memory and recall, helps him to expand his understanding of social rules and norms and encourages him to notice new and interesting facts when we're out and about.
For instance, we visited the museum on Friday and Ed took a great interest in the animals and creatures displayed there, learning the names of new animals and finding out about where and how they live. Talking about and playing with animals at home has really helped to trigger this interest.
I doubt he will ever embrace the animal kingdom in the way he loves his cars, trains and trucks, but it's really important to introduce children to new things and broaden their horizons. Introducing new open-ended, imaginative toys into play is a great way to do this.