Found a cool video of the german railway gun.

[AVGWarhawk]

Originally Posted by Task Force

probably could distroy a small city...
http://www.vincelewis.net/dora.html
The criteria that was required was that the shell had to penetrate 7 meters of steel reinforced concrete or at least 1 meter of hardened armour plate. The range required was to be over 25 miles.
it had a 7 to 8 ton shell...

The shells for Dora and Gustav, including the 1 ton charge was 17 feet long. It needed a ton of charge to send the 7 ton shell over 25 miles from the 100 foot long barrel. The shell on test firing proved to be able to penetrate 30 feet deep into earth making a crater over 90 feet feet across.

90 feet does not seem much to me for the size of the shell. Seems like a whole lot of work for 90 feet in diameter and 30 feet deep. Perhaps I'm haivng trouble visualizing this size crater.