I don't believe any of the original Maine 2 footers were ballasted with stone, with perhaps the exception of the Monson, which had scrap slate available as fill and perhaps ballast as well. Keep in the mind the period in question was 1870s to 1930s. I don't know what the standard gauge railroads were using as ballast in the teens and twenties (when the Maine Central owned the Sandy River and Bridgton lines), but my guess is generally it was dirt/gravel.