There is very little in the way of clues remaining. Apparently not electric, unless all electrical infrastructure was removed at some early point. What Mike had said about remembering two standard gage tracks, is borne out by a photo on the Mechanic Falls Historical Society Facebook page. If you scroll back quite aways on their site, a photo of a Grand Trunk freight can be found with the two tracks shown. A tank car can be seen at the right, maybe full of bunker oil? This car evidently on another spur, perhaps for the boiler house or maybe delivering chlorine or acid for the pulping operation.
The Whitin Machine Works (Whitinsville, Mass) once rostered the G.E. diesel - electric 23 tonners for that mills' operation. You can still find narrow gage rail in places and see the "engine house" with the distinctive doors and rail still bedded into the floor. A few articles were written about that two footer and the book "In This Quiet Valley" published about the mill, it's workers and production. Though there is little about the railroad within that book, it is still a very enjoyable read for its WWII content.
Those G.E diesels were very sophisticated for mill use. They were equipped very comfortably for the ooerator. The units came with in-cab heaters, windshield wipers, sanders and uncoupling controls using compressed air. In theory, an engineer could do a lot without having to dismount from the cab. In actuality, the mill cars were coupled with link and pin, and a crew did the switching, not just one man.
Initially, I had thought the Mechanic Falls industrial railroad to have been electric. The power generated from the Androscoggin River falls led me in that direction. If I find anything, I'll post it here.
