I think you mean wagon tracks. Wagon tracks are two thin lines of slag that are formed on the sides of the root pass on a pipe weld. The top of the root pass (the bead) is ground down, this grinds out some of the tracks and allows the welder to melt out the remainder with the hot pass. The bead can be ground to remove all of the tracks but that's a slow process and not used as often, and can result in burn through if you're not very carefull. Or, the bead can be only ground slightly, not taking out any of the tracks but relying on a very aggressive hot pass to remove them. A lot of it boils down to individual welders preference.
Either way they have to go as they show up clearly on x ray.


JTMcC.

I might add that you will see this when you are stick weldng on carbon steel pipe. The first pass or root pass is done with E6010 electrode. It is a deep penetrating rod so it tends to eat out the sides of the bevel and fill the undercut area along the edges of the bead with slag which creates the wagon tracks. The welder should then clean the root pass with a grinder and remove as much of the slag as possible and be sure to burn out the rest with the hot pass, which is done with E7018. I know some welders that just knock off the high spots on the root with a grinder and leave it at that and have never busted an xray. I wouldn't advise this if your a beginning welder though.
Most jobs nowadays are going with a tig root and hot pass then switching to stick to fill and cap for high pressure, or critical welds.