Eureka has 40 inch drivers and the Baldwin Narrow Gauge catalogue says the top speed for the locomotive (8-18C 4-4-0). I had an opportunity back in the early 1990s to run it up to that speed on the US Gypsum railroad. The USG used 100# rail and figured that was the biggest iron I would ever be on. So, on a level tangent I opened the throttle and let her roll.
I found the locomotive to be perfectly balanced and probably capable of going faster than 40 mpg. The limiting factor was not the locomotive but rather the tender. In spite of the heavy rails and level track, the tender set up a side sway that got rough enough to literally pitch wood over the side. That was fast enough for me! The tender carries 1200 gallons of water and it is not baffled. As a result, the water got to sloshing around considerably.
Can you imagine what it was like in 1875 when the old gal ran on #35 rail!
Dan Markoff