What does this writing on the track mean

Just a guess, but I don't think they're inspection marks but rather marks made by the MOW (tracklaying) section engineers/foremen as guides to placement of the sections of rail.

Looks like where a welder welded out a joint., or they made welds where a plug rail was installed. The 3 letters together are probably the initials of the welder and his helper, so if the rail ever breaks or the test car detects a defect they'll know who made the weld.