lines under NW loco numbers

[NW Modeling List]

OK, so I found pictures of Alco T6 #47.  One has no line and a better paint job and the other has severe lettering damage...and the line.  So time passed between the no line and line...
Are we saying that at the end of a loco service life, the centering couplers were removed and non centering couplers are replaced?
Just trying to understand.
Mike Shockley
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