Nice find. The Plymouth DDT was introduced by ATT in the late 60s. Here are a few scans of ATT material advertising the switcher. The first scan is a page from the 1967-'68 catalog showing a set powered by the locomotive. The next scan is of a 1968 order form again illustrating a set with the Plymouth DDT.
The current Model Power line has all sorts of "old school" goodies in it.
Bob