Technically, if you're not using Kato Unitrack you're violating the T-Trak specs. Practically, if you use the Atlas track you'll have trouble linking with other people's T-Trak modules, since Atlas True-Track doesn't have the same connectors as Unitrack. Also, you'll probably have to cut the Atlas track to get the lengths to match up with the standard T-Trak module sizes. In other words, if you ever plan to hook up with other people's/groups' T-Trak modules, it's best to stick to the mandated Kato track. If you're creating a private standard for yourself or your club, of course, you can do whatever you want. But in that case you probably shouldn't call them "T-Trak modules."