Bob:
As you point out Gen. Palmer of the Rio Grande may have derived his inspiration for Colorado's narrow gauge railroads during a visit to Wales. It's also worth noting that the Ffestiniog served asd the inspiration for New England's 2-footers.
Like Palmer, George Mansfield of Boston visited Porthmadoc in the early 1870's. As a result of that visit, Mansfield decided to use 2-feet as the gauge for thesed Billerica & Bedford RR in Massachusetts. The B&B was not a success and it went bankrupt in 1878. Mansfield, undaunted, took the B&B's two locomotives and some cars to Maine where he was instrumental in organizing the Sandy River RR. The Sandy River later became that state's largest narrow gauge network.