During WW2, Germany had a number of rail guns. The largest of these (mistakenly called "Gustav") was "Dora".
Dora was huge. In fact, here's a drawing of her.
http://www.aopt91.dsl.pipex.com/railgun/images/Railguns/Dora/doracol.jpg
Note, she rested on FOUR immense flatcars, straddling two tracks. Her bore was 80cm, with a shell weighing 7tons, and a barrel length around 30 meters long. Since the barrel only had vertical movement, the tracks were curved to allow for side to side movement and traverse. Her range was about 29 to 30 miles (depending on conditions she could get even further) and took a crew of 2000 to maintain and build over a six week period, and 250 to fire.
http://www.vincelewis.net/myimages19/dorashelllll.jpg
This man is standing beside one of her shells. One was enough to level a city block, or destroy even the most hardened of bunkers.
During their firing (Gustav, Dora's Brother gun was a few meters smaller) the two guns managed to level a number of locations in Sevastapol. During the attack, Gustav opened up on the coastal defense batteries. Though it had been pounded time and time again by bombers; to no avail, Gustav managed to destroy it in eight shells. Fort Stalin was destroyed later the same day, with only six shells.
Nine rounds were fired at Fort White Cliff, just off the bay, with one lucky shot hitting the ammunition locker and blowing the entire fort up. One of these shots went wild, striking a cruiser in the bay and sinking it with all hands.
With all the major targets gone, the guns were fired seven more times at nothing in particular. It's rumored one target was even a lone tank caught out in the open, but as there was nothing left after the shell would have struck it, this can't be confirmed.
It was decided later, that the guns were too unwieldy, and their use was discontinued.