Well since the railgun is AP1, you still have to roll to penetrate the armour, if a glancing hit is scored then it counts as a pentrating (thus the effects of having AP1 weapons), however against skimmers, so long it moves X" and can claim the "Skimmers moving fast rule" and you can only ever cause glancing hits.
I hope this has been helpful,
Rob
I hope this has been helpful,
Rob