You upgrade the weapon - nothing as in a visible weapon becomes visible. (compared to PD, Missile upgrades)
There is a statistic, that says that the titan has another - stats-wise another Gauss gun. (100 pen) But no projectile seems to be visible, as well as no weapon is visible from which the rail gun projectile should be firing.
I suspect that the weapon is not added, except in statistics and for making the sniping skill stronger.
That is on top of (I think unintended ?) the ability to utilize both snipe and AoE skill in 360 degree circle.
