I followed the TP&W across most of Illinois - from Watseka to Peoria - the other day and frankly was quite disappointed: not one movement anywhere. Only locomotive I saw was a UP switcher at an elevator as I neared Peoria. There was a large chemical plant near Gilman I believe, but otherwise very little business. Seemed like there were possible customers - DuPont in El Paso, for one. Am I wrong or just unlucky?
I would say that you are unlucky, and in a big way. I drive US 24 fairly regularly from Forrest, IL, to Remington, IN, and as a result see TP&W action from time to time. It isn't the busiest line, and as a result a casual drive by may not yield any train sightings at all. About half of my trips pass without seeing any TP&W movements. However, if you can find a train in this area, they are generally short (20-30 cars), and are usually good for some interesting motive power (unpatched SP tunnel motors, ex-UP GP-38s, RailAmerica odds and ends...)
I don't really know how many customers the road has in this area, though. Incobrasa Industries has a large complex outside of Gilman, where they bring in soybeans in covered hoppers (green, reporting marks BRIX) and ship out soybean oil in tank cars (black, also BRIX). Some Incobrasa hoppers are stored on a siding closer to the Indiana line, as are some seemingly virgin coalporters (reporting marks RDLX). Cargill Gilman is a large facilitiy, as far as grain elevators go, but has its own old switcher and it interchanges with the CN on the old IC line that runs north/south, and not the TP&W. CNIC also switcher out Incobrasa, though there is a fair amount of interchange with the TP&W. There are a lot of old freight cars stored on the Kankakee, Beaverville, and Southern near Webster, but unless you like old BCOL boxcars patched for EEC, don't take the side trip. I believe Effner has a small grain elevator, and there are a large number of UP and CHTT centerbeam flats stored in this area. Crescent City has a cement facility which gets CSX and NOKL (ex-CNW) hoppers. There is a large rock quarry outside of Kentland that the TP&W services, and Solae (the old Central Soya) has an elevator in Goodman. Remington has an intermodal facility, but I've never seen any intermodal traffic on the main. I'm sure there are industries in Kentland and Watseka, but I don't know what they are.
Catching the TP&W is a hit and miss proposition. I don't know what their regular schedule is, nor do I have a scanner. I'm not really a railfan, so these are just my observations from a number of trips through the area. In my experience you are more likely to catch CNIC traffic in Gilman or UP trains in Watseka than the TP&W.