Bridge Building (C-evo)

Bridge Building is a useful generally achieved fairly early n a game of C-evo. 
Its immediate value is the ability for your and similar units to build s and later railroads on s that contain s. There is a high cost in construction time: 9 or more movement points above what the tile would take without a river.
An almost immediate value is the "Overweight" feature that can be built into the design of new s that have restricted movement and double cost but exceed the standard quota and are therefore stronger than their freer contemporaries.