It's a moot point though, isnt it? The incident happened between Chorleywood & Chalfont and Latimer stations, so it was a Baker Street - Chesham through train, not a Chalfont to Chesham shuttle. The speed limit on that stretch of line is 60mph.
I am unfamiliar with the LUL stock so I couldnt comment on what speed it would have been going, but then we dont know at what point the guy fell off the train either do we? (Well, it hasnt been reported.) Was the coach empty or full? Did people actually see him fall? Was he travelling alone or with friends? A lot of unanswered questions, and arguing about the line speed on a bridge several miles from the incident seems somewhat pointless.