In answer to the orginal question, traffic between Waterville and Rigby, there hasn't been any increase in the number of trains for awhile, just the symbols change. Westbound you have road freights WARJ, NMPO, and eastbounds PONM and SDPO heading in and out of Waterville on the Freight Main, along with the Madison local going to the Madison Branch at Oakland. At Leeds Jct, westbound RIPO and eastbound PORI come off and on the Rumford Branch.
In the Portland area numerous PO locals operate, usually to Deering Jct, Brunswick, and Wells.
Two best places to visit I think are Danville Jct, where the SLR crosses and interchages, and Leeds Jct where the Rumford Branch connects with the Freight Main.