If we continued to build at 1200' per year, we could be there by 2017. However, I would imagine that unless we got a large influx of cash, that wouldn't be likely. Given the other obstacles: landslide, Cartlon brook, 218, etc. and the need to build yard trackage at Sheepscot (for the roundhouse and car barn) I think it will be at least 2020 (if not later) before we reach the south side of Head Tide Road. (If we even decide to cross 218.)
And as Mike said, a lot could (and probably will) change by then.