The former B&M/CP line to Sherbrooke has a complicated history, told in detail in Robert Lindsell's "Rail Lines of Northern New England." It was originally intended to connect the Upper Connecticut Valley --- White River Jct., specifically --- and the Canadian border. What happened from there is the complicated part, and involves the Canadian government's refusal to allow the Boston, Concord and Montreal, with which it both figuratively and literally dovetailed at Wells River, access to Montreal. Ultimately, it came under the aegis of the B&M, which operated it jointly with the CP as an important passenger route until the mid-1960's. The B&M leased the segment of the line north of Wells River to the CP in 1926. It's now part of the MM&A system. I visited St. Johnsbury many times during the '70s and early '80s, when it was still operated by the CP and was a very busy --- and beautifully maintained --- freight line linked both to the B&M and to the MEC's Mountain Division.
The MEC's line to Lime Ridge originally went from Quebec Junction, near Whitefield, to Cookshire, Quebec, where it connected with the CP, and from thence to a limestone quarry at Lime Ridge, a few miles north of there. It served several small towns and villages between Cookshire and the U.S. border (most notably Sawyerville, where there were factories and sawmills), and was a vital link for many years between Beecher Falls, Colebrook, and points south. Technically, it was two railroads under the same ownership --- the Upper Coos in New Hampshire and the Hereford Railway in Quebec. Both were leased to the MEC in 1890. Passenger service became increasingly sparse during the line's latter years, and in 1925 the MEC dropped its lease of the Canadian end and turned it over to the CP, which abandoned the segment of the line from Sawyerville south to the border. The CP continued to operate the Sawyerville-to-Cookshire segment until the late '70s, if I recall correctly. It included an impressive trestle (now gone, sadly) over the Eaton River a short distance south of Cookshire. I'm told that a large part of the former ROW is traversable by car. Just watch out for where the trestle used to be!
