Having grown up in Tecumseh in the 70s and 80s I can answer this one. Tecumseh Products last had boxcar traffic in the mid-70s. I'm not sure if they were receiving supplies or shipping out, but the spur to the loading dock wasn't used after that. They continued to receive a tank car of lubricating oil (on a different spur) a few times a year up until almost the end of Conrail operations in early '83.
Here is an earlier view of Tecumseh Products (ca. 1950) which shows boxcars inside the plant:
Tecumseh Products from WSU MVC Collection