Mike,
I think you hit the nail on the head. Lots of fast speeds have been"reported" but how many can be believed? Railroads like the SAR, with 1,800 steam engines, had dynamometer cars, engineering departments and the ability to actually test their equipment accurately. They have km posts every 1/10 km, making timing by stop watch easy. On the "Grande" an engineer could get an engine really flying along for a short distance, but there would be no way to really measure the speeed, so he might say, "it felt like I was going 60" and pretty soon, people start quoting 60mph as some sort of real event. You have to be able to check your sources. In your case, 35mph sounds like a pretty safe number.