Eureka & Palisade locomotive #4 "Eureka" (8-18C-18) was advertised in the Baldwin Locomotive Catalog as being capable of doing 40 MPH. About 26 years ago I was allowed to run it on the U.S. Gypsum railroad in Southern California. That railroad uses about #100 rail and is straight as an arrow for many miles. There is a dirt road that runs next to the railroad, so we were able to pace the locomotive and get accurate speed readings. I don't remember there being any mileposts to time our speed. Anyway, the locomotive is very well balanced. It could easily go faster than 40 mph. The limiting factor seems to be the tender. The 1200 gallons of water is not baffled and it sets up a side sway that literally pitched wood out of the tender. Not a good thing. Anyway, that side sway appears at about 35 mph and got worse as we approached 40.
On other railroads, such as the D&S, the top speeds are half that of the 40 MPH. We never get near the limit of 40 and everything goes just fine.
Dan Markoff