There's a lot of anecdotal evidence that some American steam locomotives went >120 mph in passenger service (the Milwaukee Road's F7's and the Pennsylvania Railroad's S1's, for example). I believe if you look at the PRR timetables, they had some routes that averaged 80 mph between stops, meaning they almost certainly hit a max speed at least over 100 mph. The US didn't have the same speed record contests that the Brits did though, so there's no official record.

In 1947 a law was passed requiring trains above 79mph to have some kind of automatic train stop system, so that ended most of the high speed steam service.