Hello.

Recently I have been playing with the nerfed railgun, and I admit it isn't that fun anymore. However, I think it still outmatches mobile artillery and is a nice all rounder(helicopter, SAM, missile, armor) killer.

In general:

Railgun should preferably be used in open ground, but Cities/Suburbs and Desert/Tundra do not provide negative modifiers.
Using Railgun in Forests, Jungles, or Mountains(unlike normal arty) will reduce your damage by 50%.

Tier 1 Guide:
Building requirement: AB2, AI 1, Secret lab 1

Railgun should be used as normal artillery, bombarding enemy units from afar without taking damage. Railgun should not be used in close combat.
Railguns can also be used to hit corvettes shelling your cities... Corvette deals 1 damage: Railgun deals 8 back.
Range: 50

Tier 2 Guide:
Building requirement: AB3, AI 1, Secret lab 1
Railgun must be paired with some kind of radar to be effective anti air, it cannot shoot down enemy planes if it cannot see it. To that note, the railgun's anti helicopter features are primarily defensive, as rotary-winged aircraft are hard to track by radar.
Terrain modifiers remain, but now you can airlift it anywhere within 5000km. The Railgun is slightly buffed now and deals anti air damage approximately equivalent to a mobile anti air. Can shoot down Cruise Missiles.
Range: 50
Anti Air: 75
Tier 3:

Building requirement increased to AB5 + Secret Lab 2 + AI 1

Railgun now deals the damage of nearly a SAM, as well as 7.0 helicopter damage. 2 Railguns in a city can shoot down an inbounding Nuclear Ballistic Missile. More buffs to anti armor/anti ship.
Range increased to 75 for artillery, 100 for Anti Air.

Air Assault feature added: if your enemy only uses meelee combat, you can airdrop them behind enemy lines where they can obliterate the front line.
Also effective when paired with traditional air assaults(Tank Destroyer, Airmobile, Spec ops).

Can now be airlifted to anywhere in the world without needing to refuel.

Hope this helps!