Help with Additional Lego City Train Parts

Hello! I recently got my nephew the Explorers' Arctic Polar Express Train and he loves it! I got him a bunch of extra tracks to go with it so he could do crazy designs to his hearts content. I'd like to get him more parts so he can make additional train cars, but I'm really at a lost. There's nothing on the lego website. I have found some ebay listings with "Lego City Train Wheels" but I'd also like to get him some connectors that will work with his set, some chassis to build on, and maybe...