Nathan Algren is a pretty good fighter and not someone that you want to mess with. If he gets close to his target then that is usually game over for the opponent. Still, Train is a lot faster than any human so this wouldn’t apply to him and what’s more, he has the Railgun which covers the long range options. Nathan can’t escape and he can’t outfight Train. It’s over for him. Train wins.
Suggested by JHG195 It looks like it is time for Stannis to return to the battlefield. He is a tough fighter so this won’t be an easy win for Nathan. Still, few can match Nathan in terms of pure sword skills and the guy did train an entire army. I just don’t think Stannis has the speed or stamina to keep up with Nathan and that will be his downfall. He simply doesn’t have the skills to win this fight. Nathan Algren wins.