According to John Tatko, owner/manager of the Sheldon Slate Company's Monson operation (former Portland-Monson Slate Co.), the Portland-Monson did not use electric locomotives in its quarries. Slate was moved from tunnels into the main shaft by means of winches.
John did think that electric locomotives had been used by the Monson Maine Slate Company in some of its tunnels.
I have also heard that MMSC sometimes used horses in its quarries to move slate from tunnels. The horses, wearing blinders, were lowered into the main quarry by derrick. One such horse, at the end of the work day, reportedly would go on its own to the daylight end of the tunnel and wait to be lifted to the surface.