It means there's a derail at that location.
Here's a fun read:
http://www.railroadforums.com/forum/showthread.php?t=15905
Ed
Like this one at my module Diamond Valley
Wolfgang
Pueblo & Salt Lake RR
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D denotes derail (not derailer).
If it is set to derail equipment coming out of the siding onto the main, it is for the use of the train crews to keep cars placed in the siding from rolling out.
If it is set to derail equipment going into the siding then it is to protect MofW or Mechanical people from having a train or equipment enter the track.
Dave H. Painted side goes up. My website : wnbranch.com
On our excursion to Bakersfield this last weekend, we passed a number of sidings with a purple sign stating "D", as well as what looked like derailer rails across the tracks, but also some fairly long sidings.
Is this what that means and what is the main usage? I figure it may have to do with MOW equipment?