8 July 2018 - As Tom Strahan runs after his big brother Jack, he shows the same desperation to keep up that any little brother has, but his legs hit the ground with slightly more caution than most.
The five-year-old makes himself heard with his tiny voice, yelling 'train' whenever one comes rattling past his local playground.
But, medically speaking, he shouldn't be able to do any of these things.