The low grade line was built to cut the grade on the south side of Richmondville for what was then (early 1900s) a predominantly loads-north railroad. Anthracite was mined in PA, principal markets were in NY and New England, so the northbounds tended to be loads, thus heavier than southbounds.
Basically, the line cut the grade by winding around on the valley floor a little more than the main, climbing the same vertical distance over a longer horizontal distance. This route missed both of the towns (Worcester and E Worcester) and thus had few if any shippers.
By 1964, the anthracite biz had largely evaporated as people changed to heating oil for home heating. (My parents, never technological leaders on issues like this, made the switch in 1962). Heaviest train on the route now was the paper train, RW-6, loads southbound. And the trains were all diesels to boot, which had better low-speed tractive effort than the steamers that the line had been designed for.
So as mentioned earlier, the low grade had few shippers, and its reason for existence was largely gone. Not a hard call for abandonment.
Challengers definitely ran over Richmondville into Mechanicville--one blew up on Richmondville hill in the 1940s. Jim Shaughnessy's book has lots of pix of them at least as far north as Mechanicville. I believe I've heard reports that they went up to Whitehall, but I can't verify that.
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