The cool thing about steam locomotives is you can determine their speed just by the exhaust beat, if you know the driver diameter. And in the year 2020, there's nothing you can't find out on the Internet.
A fellow in the comment section of that video claims 40 km/h. My tap metronome tells me those 34-inch drivers are doing 270 RPM, which comes out to ~27mph or ~43km/h.