This is NOT the Portland & Rumford Falls,but the original Androscoggin RR.. The Portland & Rumford Falls left the MEC mainline at Rumford Jct.,which is just off Hackett Rd. in Auburn(and still a name of a Guilford Passing siding there). Tracks ran to the L_A airport, crossed the SLR twice,over through Poland area,into Mech Falls,crossed SLR again,and meandered up through Hebron,Buckfield and Canton area.
It WAS the original line to Rumford,but once a connection was bulit by MEC from Livermore Falls(present route) it began to decline. By 1947,no more through freights,only segement open was from Canton(where it joined the Rumford Branch from Livermore Falls,down to Poland Corner(stationthere is just off Rt. 26 and is beautifully restored as a home). Tracks all torn up in 1952,except for a short segment to serve a large tannery in Canton. Tracks abandoned in 1970 there.
Segment of track you saw,was the part of the original line from Brunswick to Leeds Jct. then to Farmington.(they built from Crowley's to Lewiston Lower later on!). The segment from Crowley's to Leed's Jct. was abandoned in early 1930's,and biggest user were 2 large mills in Sabbattus. Most of the tracks were ripped out,except for the small section used to store cars in the 1930's,and could be used for switching Polar Chemical at Crowley's, a rail user until the 70's.
Bud