It's been claimed at least as far back as H. T. Crittenden writing in the Bulletin of the Railway & Locomotive Historical Society in 1932 that George Mansfield visited the Ffestiniog Railway in the early 1870s and was then inspired to build the Billerica & Bedford, which then spawned the Sandy River RR et al. However, Donald Ball's recent (2011) book on the Billerica & Bedford casts serious doubt on this.
Essentially, Ball was able to account for Mansfield's activities during the period in question, and between the death of his first wife in May 1870 and his second marriage in October 1871 as well as various business ventures and other periods of employment between 1870 and 1872, it doesn't appear he would have had the time to sail from Boston to the UK, visit Pothmadog, and then sail back to Boston, even if he'd been able to afford such a trip. Mansfield may have been a man of big ideas, but for a busy 30-something guy (he was born in 1839) of modest means it would probably not have been possible.
Ball thinks it's much more likely that Mansfield simply encountered decriptions of the Ffestiniog Railway in narrow gauge promotional literature by General Palmer, Fairlie, and Charles Spooner himself, and found his inspiration in those without ever having been to Porthmadog in person. Significantly, Ball notes that in a series of articles on the history of narrow gauge railroads that Mansfield published in 1881 in the Phillips Phonograph newspaper in Maine, Mansfield refers to descriptions of the Ffestiniog Railway by Palmer and other visitors but makes no mention of having been there himself.
-Philip Marshall