While it doesn't appear in the Van Buren tech demo, it is still present in the game files and can be spawned with the console commandCreateEntity Weapons15mm_Artemis_railgun.wea, but remains a non-functional weapon.
Even when fully loaded, the gun may not fire. The player will simply reload the gun and face the intended target, never firing.
J.E. Sawyer's Fallout PnP doesn't mention Artemis and 15mm EC ammunition as present in its world (15mm EC reference was accidental). Rail guns were mentioned in connection with its technology level and Advanced Research perk only.