The new puzzle, courtesy of @sherriburroughs:
Dave is, recklessly, walking across a railway bridge1 and is three-eights of the way across when - disaster! He hears a train coming. It's the kind of train that travels at a constant speed of 100 km/h.
If he runs towards the train, he gets off of the bridge just in time; if he runs away from the train, he also gets off the bridge just in time.
How fast does he run?