The bridge builder is a tool that can be added to a cart to allow it to place down blocks to act as bridges for tracks
This is best used in combination with the Railer on a cart to create a minecart track
if you are using this underground then you should use this with a coal engine and at least a reinforced drill
this can end up with it burning in lava though
Use this if you are busy with other things (ex. building something big, using things from mods like Buildcraft, Tekkit, Flans Mod, PneumaticCraft etc.)