the coaches are hollow inside so you can fit the any big items inside the shell. As for the thomas, the builder must cut a big hole on the thomas' chassis.
My Trackmaster/Tomy/Plarail Photo Gallery Page (over 600+ photos and still under construction)
https://www.flickr.com/photos/183311600@N03/
https://www.flickr.com/photos/183311600@N03/