To begin, Amtrak's Vermonter runs southbound Monday-Friday as #55 (#57 Sat/Sun) and is scheduled to stop in Northampton at 2:01pm, while the Monday-Friday northbound #56 (#54 Sat/Sun) is scheduled to stop at 3:57pm. CAVEAT: The southbound Vermonter ALWAYS runs late by at least 15 minutes to a half-hour, so don't take the schedule as gospel. In fact, if you have a Smart Phone, get the Amtrak App. Pan Am doesn't usually run on weekends, so don't expect anything those days, but Monday-Friday is fairly active. EDPL (East Deerfield-Plainville, CT) normally runs on Monday and Thursday, while the northbound PLED normally runs on Tuesday and Friday, however they ran an EDPL last night around 4-5pm which is unusual. Look for EDPL on Monday and Thursday afternoons parked on the secondary track near exit 18 I-91, as they often park it there until the northbound Vermonter goes through. There's also the Local which is designated either ED-2 or ED-4, ED-2 usually being the designation. The only customer in Northampton that I'm aware of is PCA, which gets boxcars and is switched on an as needed basis. Basically, it's catch-as-catch-can when railfanning the Connecticut River Line.
Hope this helps!