[QUOTE]Originally posted by hedkikr:
The way I see it, the train example doesn't apply because a train w/ no acceleration is sitting still. It had to accelerate from 0 to "X" mph. The fact it reached "X" mph & remained constant doesn't disqualify the formula. The impact on your body would be the same @ the instant the train hit "X" mph or an hour after it hit "X" mph (remaining constant).[/QUOTE]
A train at any constant speed, be it 0mph or 100mph, will not be accelerating (even though it needs a force to equally counter various friction forces).
You are right about the impact being the same if you're hit while it is at constant velocity, or if it reaches X velocity during a period of acceleration. At the moment of impact it's kinetic energy would be the same, or it's momentum would be the same. If the impact conditions were the same in both examples, then the acceleration you experience would be the same too.