From what I understand, it is irrelevant whether you choose to defend or attack a rail. Whatever the outcome, the damage is being applied to the defenders, and not the attackers.
I've been monitoring this site for 2 days now: http://deathsnacks.com/wf/
Every single conflict on there had the defender rail health going down, but the attacking rails are all at 100% health. I assumed the site was broken, but it's not. The system is broken. Attackers simply need to deploy a rail, and any missions played for/against will always damage the defenders.
Basically, a serious and a game breaking bug