The Railway gun the Germans used to shell Paris in WWII

There are two ways, one is you can go to a power station (forget which one) and you can get a schematics and build it yourself. Or you can go to a "random encounter" area and you will meet a guy by himself with a railway gun, he will attack you.

The railway-gun Schwerer Gustav which could fire an 31 inch , seven ton shell twenty-three miles away .

The massive Schwerer Gustav gun (80 cm (31 in) was the largest artillery piece (Railway Gun) to be used in combat deployed by Germany .

The Dora and Gustav railway guns where highly irrelevant but still highly effective due to its propaganda and psychological value.
The shear sound and sight of such a gun and its effect on the ground must have driven the enemy crazy with fear...