Some GEarth surfing between West Concord and South Acton will show the ghosts of 2 trailing-point turnouts on the EB/#2 track a bit east of the Parker Street grade crossing.
"Powder Mill" is also a frequent name for in the area for roads -- Rte. 62 for one -- and also a shopping plaza. "American Powder Mill" as a Wikipedia entry gives a brief history of what was once both a large and dangerous operation. If the Wikipedia entry is correct, gunpowder production stopped in 1940, curious since WW II had already started in Europe, but given the persistence of names in railroading, it's not surprising that "Powder Switch" stuck as a designation.
As usual, there's probably a lot more detail.
"A gray crossover is definitely not company transportation."