The Pain Train perk allows you to sprint into enemies and inflict damage. Wearing Power Armor increases its effectiveness, and may even grant access to Power Slam - an area-of-effect jump attack.”— Fallout 4 loading screen hints
The first rank of Pain Train, allows you to damage and stagger enemies by sprinting into them while wearing power armor. The second rank makes this damage and stagger even more substantial, and the third rank causes massive damage and knocks enemies down.
- Robots and oversized enemies are immune to stagger, somewhat reducing the effectiveness of the perk.