The TD3 page is the biggest source I've found, with the dispatchcentral Yahoo! group being second.
Some of the "for sale" territories deserve a second look. I picked up the CSX NI desk one and have been quite happy with it. There's a PDF guide to accompany it (seperate download from a different site) that does an excellent job at describing the territory, operations, and how best to set up TD3 to act as prototypically as possible.
Overheard in NY Penn: "All aboard! Get on the train if you're coming with us!"