WADR,
Are your experiences in WELDING, TRACK, OR BOTH?
Mine is BOTH.
IMO, IT IS A BROKEN RAIL.
The break is just to the right of a weld (likely a weld made in the field).
Likely, the weld may have been done incorrectly POTENTIALLY AFFECTING THE TEMPER, and/or not GROUND to the “profile”, or to compensate for “mis-match” of the rail-heads.
Constant pounding of steel wheels on steel rails, WHERE THERE IS NO RAILROAD TIE TO SUPPORT IT, could cause cracks, and fluctuations in temperature may well result in a break.
The rail HEAD becomes WORK-WORN AND BRITTLE, and the split runs through the WEB AND THE BASE OF THE RAIL.