I had never seen a sign like this before. So, naturally, I had to investigate. It seems the railroad crossing just ahead has a large hump; large enough to create a problem for a vehicle towing another vehicle.
Who'd a thought that would be a problem? Evidently it became a problem. Thus, the powers that be put up a sign. But try to figure out that sign going 35 miles an hour. Especially if you've never seen one before.