One reason the U.S. Army is moving so rapidly to develop "Medium Brigades" (that do not rely on the 60 ton M-1 tank) is that the M-1 was designed for use in Central Europe, where most of the bridges were built to handle a 60 ton tank. As a result of that, the army didn't invest in a lot of portable bridges that could carry a 60 ton tank. Ironically, the Russians did. The Russian PMP Ribbon Bridge is a marvelous piece of engineering. Not only can it carry a 60 ton tank, but the PMP equipment is so well designed that it can be assembled at the rate of about 7 meters a minute (a 140 wide river could be bridged in 20 minutes.) The United States also has similar heavy bridging equipment, but it's owned by American railroads, to be used when railroad bridges are down due to flood or accident. As the American army discovered when it moved M-1 tanks into the Balkans in the 1990s (as part of a peacekeeping force), if the local bridges can't handle 60 ton tanks (and most Balkan bridges could not), your armored force moves a lot more slowly. You'll hear more about this issue if U.S. tanks are sent to an area with rivers, and bridges that can't handle 60 ton tanks.