The Germans had several rail guns during the second war, which all were descendants of the the 60cm Karl the Great WWI gun (which shelled the outskirts of Paris during the waning days of WWI), with the largest being Dora, an 800mm howitzer that launched 48 shells in anger during the second world war, and amounted to being an early weapon of mass destruction.
This thing took a crew of 1450 men 4 to 6 weeks to assemble and service the weapon and 500 just to shoot it. It had it's own airplanes and intelligence units assigned to select and perform BDA targets. They were designed with the intent of engaging Maginot line forts, and were made superfluous for this purpose when tactics changed (see Guderian) and they sent a 10000+lb HE
shell, or a 16000+lb penetrator shell (for bunkers and forts) a distance of 25 miles. These were the first true and effective mobile superguns.
Here's a website for Dora
and here's a website describing a 280mm gun that was also mounted on a rail car by the Germans.
[Edited 2005-12-08 22:10:39]