The Priming Track will light the fuse of any TNT Cart that passes over it. Fuse length can be adjusted by right-clicking on the rail with a Crowbar. Fuse length is measured in 1/20ths of a second (ticks). This technique can be used in a TNT Cart-Cannon together with the Launcher Track. You can pair this up with a Priming Track to create an effective TNT cannon.