The Undercutter Cart is a special type of minecart from the Railcraft mod. This minecart is used to replace the blocks under your tracks, as well as the blocks on either side, without having to break tracks. If linked to a Chest Cart using a Crowbar, all of the blocks and items broken will be placed into the Chest, rather than dumped onto the ground, and the Undercutter will use the blocks inside the Chest as a ballast material, if defined in the GUI.
The Undercutter Cart has 4 sections: Pattern, Under, Stock, and Sides. Pattern is used to define the blocks that are valid to replace, placing blocks in here will use ghost blocks. Under is used to define what blocks are valid to use to replace the blocks underneath the Tracks, placing blocks in here will use ghost blocks. Sides is functionally identical to Under, only it is only valid on the blocks next to the "Under" blocks. Stock sections are where you place the blocks you want to use as the Under or Sides, this will not use ghost blocks, as it needs to use the items to place them.