I think NMC does have rail cannons, the two dual cannons on the side/shoulder of the ship (Battle-7 type).
Battle-7 is probably got the worst action record in term of show, it was shown only during transformation and firing its Macross Cannon. We never see any other weapons on it, even it was shown to have several missile launchers popping up, we never know where the location of those launchers are.
I was even pissed of when Battle-7 fold in and ram the nearest Varauta ship (even Max said there's not enough time to transform and fire the Macross Cannon).
If all the NMC Battle class are unique, I wonder what Battle-1 look like, maybe I just grab one of Miyatake concept design and designated it Battle-1 to Battle-6