The Piperunner is an Indirect-Combat Unit introduced in Advance Wars: Dual Strike. As its name implies, it is the only unit capable of moving on Black Hole's Pipelines, but it can also move onto Factories as well for Fuel, Ammunition and HP.
Despite the limited terrain that it can move on, a Piperunner is capable of moving up to 9 Spaces in a turn, but like all the Indirect Combat Units of its time it cannot move and attack on the same turn. Its firing range is 2-5 spaces away, capable of hitting any unit within its range except for a Submarine that has dove underwater and a Stealth Fighter that is in hiding.
To unlock the Piperunner unit, the lab map has to be found in the mission Snow Hunters, then capture 15 Properties in the subsequent mission Spiral Garden to gain access to the Piperunner for the rest of that Campaign and all other game modes. It costs 20,000G to build a Piperunner unit.
Despite their limitations, a Piperunner can be very useful at repelling enemy attacks. They still need protection from direct attacks, like all other Indirect Combat units.
If your opponent has Piperunners charging at you, breaking a seam in the Pipeline will stop that charge cold, limiting how far the Piperunner can go.