The railroad line that includes the Onawa trestle was built by the Canadian Pacific in the late 1880's and opened in 1889. According to "The Rail Lines of Northern New England," by Robert M. Lindsell (a "must-have" book from Branch Line Press), the Onawa trestle is 1200 feet long and is 130 feet above the low water mark of Ship Stream. There is another substantial trestle at Wilson Stream, but that one is nowhere near as impressive.
Unlike the Intercolonial line (later CN) that skirted the top of Northern Maine, the CPR line went directly across Maine, bound for St. John and Halifax. Being shorter than the line that skirted Maine, it was known as The Short Line. Crews ran from Megantic to Brownville Junction and Brownville Junction to McAdam, running alternately on these two assignments, as the mileage were slightly different. My grandfather was based in Brownville Junction.
Because the line crossed international boundaries, there were customs stations at the ends, one in Jackman, Maine, for example. Sleeping car doors were sealed at the borders so that sleeping car passengers need not be disturbed by customs inspectors. Their cars were treated as rolling pieces of Canada.
Returning to the subject of the trestle, there is a great story in "All Aboard for Yesterday" (Down East Books) about Axel Carlson, who was blown off the bridge in 1912, fell 77 feet onto an ant hill, and lived to tell about it for 60 years. The same article says that the orginal bridge was wood, built in 1887. This was replaced about 1896 by a steel span, which in turn was replaced by the present structure in 1931.
More than you ever wanted to know, I'm sure