qaccy
Emperor
rails ignore defensive bonuses AND terrain, that's why they can shoot at stealth and flying units.
They ignore defensive SKILLS, not bonuses. If a unit has a defense bonus against artillery units, it will take reduced damage from a rail gun's attack. If a unit has a defense bonus on a specific type of terrain, it will be ignored. Beyond that, rail guns also ignore flight, stealth, reactive armor, and dug in. Perhaps chivalry and call of duty as well, but those are such small bonuses it's hard to tell.
To reiterate, rail guns do NOT ignore defense bonuses that units have against artillery. They ignore everything else.