Look what I ripped out of the Outpost 2 manual.
Railgun
Common Metals Cost: high
Rare Metals Cost: none
Area of Effect: very small
Range: long
Rate of Fire: medium
Concussion Damage: medium
Penetration Damage: medium
RPG
Common Metals Cost: high
Rare Metals Cost: none
Area of Effect: very small
Range: long
Rate of Fire: very high
Concussion Damage: medium
Penetration Damage: low
Damage Types and Armor
Many vehicles and structures in the game have armor to protect them from disaster and attack. The damage done by weapons and disasters may be either concussion damage or penetration damage; many weapons do both kinds of damage. Concussion damage, which results from the application of raw force, can be partially reduced by armor. Penetration damage, which results from the piercing of shrapnel or other projectiles, slices right through armor. Armor comes in three different levels: light, medium, and heavy.
While they deal the same amount of concussive damage, the RPG benefits from faster rate of fire and fires "indirectly", while the Railgun benefits from its ability to deal better penetrating damage.
Both the Railgun and the RPG gets my support since the penetration damage is ideal for eliminating Lynxs, non-combat units, and even Arachnids when you do not have a Acid Cloud vehicle. Though I should not forget that a Railgun projectile can pierce through the armor of a Panther. I believe that the RPG's explosion will be reduced by a Panther's medium armor, but the Railgun can easily rip through it.
So I did not vote on the Railgun not only because it is "cool". When compared to a RPG, the Railgun can take on any medium armored buildings/units any day.