During WWI Germany actually built two of these as prototypes...
"Kolossal-Wagen"
This super heavy breakthrough tank was designed for planned service in 1919. Two prototypes were built, but never tested and destroyed after the war.
The K-Wagen
Plans called for the K-Wagen to be able to be broken into four parts for rail transport and was to be operated by a crew of 22 men. There was no information as to what it may have weighed or what the armament might have been.