Super simple to do: Get Mk2 power armor, throw in x3 batteries (mk2) and two fusion reactors. Get some mk2 shields in there, charged up, and then have a train run at you for around 60% speed. Just make a simple turn with the rails and stand on the rail as it's coming around.
Do a quicksave before starting turning the train to auto from manual, so you can adjust where you're standing. Might need a few tries before you get it. I got it in about 4 tries.