rails ignore defensive attributes
For clarification, rail guns ignore TERRAIN bonuses. A drone swarm takes reduced damage from a rail gun thanks to its artillery bonus, but the rail gun will deal the same damage to it regardless of whether it's on plains or in a house (where it has a defense bonus from the terrain).
However, since rail guns have a bonus against light units (and larger than what dragons get), the dragon drone ends up not being very effective against them.
As a third point, based on what the AF units look like so far the AF artillery is going to have a bonus against heavy and ranged units. Whenever it happens to be released, that is. Heavy>ranged>light>fast>artillery, with each unit having bonuses against the next two in the line (artillery wraps back around to heavy).