Saw this on the routes and decided to have a long hard workout as a birthday run.
Conditions were pretty ideal, 30F, light wind. Bridges were iced over, the first main bridge over the Connecticut River especially and there were about 3-4 short sections of frozen melted snow that really slowed the pace out by Amherst College, but these were only 300-400 yards long at a time. Maybe a mile in total.
The route started and finished at Woodmont Road in Northampton, turned around at Warren Wright Road at the end of the parking lot, the GPS track shows me turn slightly earlier for some reason. The mile posts show the start point is about 0.7 miles into the rail trail, but maybe it crosses roads or parking lots so I just started at the point highlighted on the FKT route that was posted by Andy. I had it about 10.6-10.7 miles in length. I'd imagine GPS signal may be less accurate once the trees get their leaves back. There did not seem to be any water fountains to use if it was ran in the summer. I carried about 30 oz of gatorade in a waist pack and handheld and had 3 gels.
It's a nice route, surfaced and cleared in the winter. Slightly undulating but nothing steep. There's a few roads to cross but only the one just before the Connecticut River crossing was busy.
I parked at the back of Walgreens and used their rest room and it was 0.1 miles down the rail trail to the start location. There's also a few fast food joints there but the McDonald's rest room was a threat to humanity and the Taco Bell locked due to Covid. There are various possible start locations along the route, I opted for Northampton due to toilet access, ease of travel and also you end with 5-6 miles of gradual downhill. But as it is an out and back any start point is fine.