The Bridge Laying Tank; Then and Now
Ever since the dawn of mechanized warfare in the early 1900's, there has always been a need to get vehicles across terrain that is just horrible. Then there came the caterpillar track system, which could go through relatively thick amounts of mud and other previously impassable terrain with ease. Still, these systems were complicated, heavy, and get bogged down just too often. This is where the Bridge Laying Tank comes into play. In the rainy last days of WWI, a British Mk. 1 medium tank lugged a wooden scaffolding bridge across the thick mud to allow infantry to cross. This is unofficially the birth of the bridge laying tank.
In WWII when the allies land in Normandy, France as phase 1 of Operation Overlord, the M4 Sherman tanks of the American 3rd Armored Division need to cross deep trenches and ditches that were dug by the retreating German Infantry. Shermans with crane arms and a steel bridge instead of a standard turret are called to resolve the issue. They lay the temporary bridges to get the rest of the invasion force across.
Now, modified M1A1 Main Battle Tanks are the American way of fixing the century old problem. They like the M4 Shermans of olde sport a removed turret to carry the crane and bridge. In Iraq and Afghanistan, Al Quaida blows bridges and freeways to prevent the armor from rolling in. No problem for the Army Corps of Engineers, them and their Bridge Layers get the convoys moving again.