I believe Single Tracks were the 9xx series and were serious mountain bikes with 26" wheels. Better tubing and components. Multitrack was their 7xx hybrid bike with 700c wheel size, more upright riding with mountain bars/brakes/shifters on a "road bike" type frame. Mountain Tracks were their 8xx series bikes (sometimes called "Antelope"), and were more trail and light mountain biking rigs with 26" wheels. Not as aggressive position as on their 9xx series, "lesser" quality tubing and components vs the 9xx series.
If you go to: Vintage Trek Bikes- Information on Steel Road Bicycles made by the Trek Bicycle Corporation, bike they have catalogs and you can compare the different ones.