No, the claim "early in the development of trains people were sure it would be physically impossible for humans to travel over 30 mph or so" cannot be correct, because people had been traveling faster than 30 mph on horse back for at least centuries. There's no way anyone can think was true given that they had many counterexamples.

The article you sited says that some people thought there might be issues at 50 mph. Evidently there were some people who thought this, but that doesn't say anything about how wide-spread that thinking was.