Cecilia Townley, a teacher from Surrey who is visiting for the Games with sons Frank, 11, and Vincent, 9, got chatting to Jamaican fans on the Subway.
It was only when she spotted the name 'Bolt' on an identity badge that she discovered the man was the sprinter's father, Wellesley.
Cecilia said: "I looked at his badge and jokingly said 'you're not a relation are you?' He said 'actually, I'm his dad'. His mum was there as well with all the family."